Menalled UD, Pethybridge SJ, Pelzer CJ, Smith RG, DiTommaso A, Ryan MR. High Seeding Rates and Low Soil Nitrogen Environments Optimize Weed Suppression and Profitability in Organic No-Till Planted Soybean Agroecology applied to arable crops View project Crimson Clover Accessions Trial View project. Frontiers in Agronomy [Internet]. 2021;3. Publisher's Version
Pearsons KA, Rowen EK, Elkin KR, Wickings K, Smith RG, Tooker JF. Small-Grain Cover Crops Have Limited Effect on Neonicotinoid Contamination from Seed Coatings. Environmental Science and Technology [Internet]. 2021;55 (8) :4679–4687. Publisher's VersionAbstract
Neonicotinoids from insecticidal seed coatings can contaminate soil in treated fields and adjacent areas, posing a potential risk to nontarget organisms and ecological function. To determine if cover crops can mitigate neonicotinoid contamination in treated and adjacent areas, we measured neonicotinoid concentrations for three years in no-till corn-soybean rotations, planted with or without neonicotinoid seed coatings, and with or without small grain cover crops. Although neonicotinoids were detected in cover crops, high early season dissipation provided little opportunity for winter-planted cover crops to absorb significant neonicotinoid residues; small grain cover crops failed to mitigated neonicotinoid contamination in either treated or untreated plots. As the majority of neonicotinoids from seed coatings dissipated shortly after planting, residues did not accumulate in soil, but persisted at concentrations below 5 ppb. Persistent residues could be attributed to historic neonicotinoid use and recent, nearby neonicotinoid use. Tracking neonicotinoid concentrations over time revealed a large amount of local interplot movement of neonicotinoids; in untreated plots, contamination was higher when plots were less isolated from treated plots.
Billman ED, Souza IA, Smith RG, Soder K, Warren N, Teixeira FA, Brito AF. Winter annual forage mass‐nutritive value tradeoffs affected by harvest timing. Crop, Forage & Turfgrass Management. 2021.
Smith RG, Warren ND, Cordeau S. Are cover crop mixtures better at suppressing weeds than cover crop monocultures?. Weed Science. 2020;68 (2) :186–194.Abstract
Cover crops are increasingly being used for weed management, and planting them as diverse mixtures has become an increasingly popular strategy for their implementation. While ecological theory suggests that cover crop mixtures should be more weed suppressive than cover crop monocultures, few experiments have explicitly tested this for more than a single temporal niche. We assessed the effects of cover crop mixtures (5- or 6-species and 14-species mixtures) and monocultures on weed abundance (weed biomass) and weed suppression at the time of cover crop termination. Separate experiments were conducted in Madbury, NH, from 2014 to 2017 for each of three temporal cover-cropping niches: summer (spring planting-summer termination), fall (summer planting-fall termination), and spring (fall planting-subsequent spring termination). Regardless of temporal niche, mixtures were never more weed suppressive than the most weed-suppressive cover crop grown as a monoculture, and the more diverse mixture (14 species) never outperformed the less diverse mixture. Mean weed-suppression levels of the best-performing monocultures in each temporal niche ranged from 97% to 98% for buckwheat (Fagopyrum esculentum Moench) in the summer niche and forage radish (Raphanus sativus L. var. Niger J. Kern.) in the fall niche, and 83% to 100% for triticale (×Triticosecale Wittm. ex A. Camus [Secale × Triticum]) in the winter-spring niche. In comparison, weed-suppression levels for the mixtures ranged from 66% to 97%, 70% to 90%, and 67% to 99% in the summer, fall, and spring niches, respectively. Stability of weed suppression, measured as the coefficient of variation, was two to six times greater in the best-performing monoculture compared with the most stable mixture, depending on the temporal niche. Results of this study suggest that when weed suppression is the sole objective, farmers are more likely to achieve better results planting the most weed-suppressive cover crop as a monoculture than a mixture.
Mortensen DA, Smith RG. Confronting Barriers to Cropping System Diversification. Frontiers in Sustainable Food Systems. 2020;4.Abstract
There is no shortage of data demonstrating that diversified cropping systems can sustain high levels of productivity with fewer external inputs and lower externalities compared to more simplified systems. Similarly, data exist indicating diverse cropping systems have greater capacity to buffer against and adapt to weather extremes associated with climate change. Yet, agriculture in the US Corn Belt and other major crop production regions around the world continues to move toward simplified rotations grown over increasingly large acreages. If our goal is to see more of the agricultural landscape made up of diverse agricultural systems and the ecosystem services they provide, it is critical we understand and creatively address the factors that both give rise to monocultures and reinforce their entrenchment at the exclusion of more diversified alternatives. Using the current state of farming and agriculture policy in the US as a case study, we argue that a pernicious feedback exists in which economic and policy forces incentivize low diversity cropping systems which then become entrenched due, in part, to a lack of research and policy aimed at enabling farming practices that support the diversification of cropping systems at larger spatial scales. We use the recent example of dicamba-resistant crops to illustrate the nature of this pernicious feedback and offer suggestions for creating “virtuous feedbacks” aimed at achieving a more diversified agriculture.
Lowry CJ, Bosworth SC, Goslee SC, Kersbergen RJ, Pollnac FW, Skinner RH, Warren ND, Smith RG. Effects of expanding functional trait diversity on productivity and stability in cultivar mixtures of perennial ryegrass. Agriculture, Ecosystems and Environment. 2020;287 :106691.Abstract
Cultivar mixtures can provide a host of beneficial agroecosystem services in annual grain crops; however, it remains unclear whether these same benefits apply to perennial forage cropping systems, or the degree to which beneficial effects depend on the functional trait diversity of the mixtures. We conducted a field experiment across four locations in the Northeast US in which we grew perennial ryegrass cultivar mixtures varying in cultivar number and range of expression of three functional traits (winter hardiness, heading date, and extended growth) and assessed the effects on dry matter yield and inter-annual yield variability. Trait ratings supplied by the seed company were related to observed variation in perennial ryegrass productivity and/or stability at both the individual cultivar and mixture levels. Winter hardiness was associated with greater perennial ryegrass cumulative biomass, and lower interannual stability. In contrast, extended growth was associated with lower cumulative biomass, and both extended growth and later heading date were associated with greater interannual variability. Overall, cultivar richness was negatively associated with perennial ryegrass biomass and stability; however, the best performing mixtures performed as well as the recommended cultivar for the region. When comparing mixtures with equal cultivar richness, functional trait diversity measured as the additive trait range—the summed range of the three traits associated with the cultivars in that mixture—was positively associated with biomass production and over-yielding, but not interannual variability. Cultivar mixtures of perennial ryegrass can lead to improved forage production when specific functional traits are optimized within mixtures. Our results support the growing understanding that efforts to ecologically intensify agriculture through enhancement of crop diversity are more likely to succeed when they explicitly consider the functional traits of the crops involved rather than solely numbers of cultivars or species.
Coble AP, Contosta AR, Smith RG, Siegert NW, Vadeboncoeur M, Jennings KA, Stewart AJ, Asbjornsen H. Influence of forest-to-silvopasture conversion and drought on components of evapotranspiration. Agriculture, Ecosystems and Environment. 2020;295 :106916.Abstract
The northeastern U.S. is projected to experience more frequent short-term (1−2 month) droughts interspersed among larger precipitation events. Agroforestry practices such as silvopasture may mitigate these impacts of climate change while maintaining economic benefits of both agricultural and forestry practices. This study evaluated the effects of forest-to-silvopasture (i.e., 50 % thinning) conversion on the components of evapotranspiration (transpiration, rainfall interception, and soil evaporation) during the growing season of 2016. The study coincided with a late-summer drought throughout the northeastern U.S., which allowed us to also evaluate the effects of forest-to-silvopasture conversion on drought responses of multiple tree species, including Pinus strobus, Tsuga canadensis, and Quercus rubra. In the reference forest and silvopasture, we observed declining soil moisture and tree water use during the drought for all three tree species. However, the decline in P. strobus water use in response to declining soil moisture in the silvopasture was not as steep as compared with the reference forest, resulting in greater water use in the silvopasture for this species. In contrast, we did not detect different water-use responses between forest and silvopasture in T. canadensis or Q. rubra. This suggests that forest-to-silvopasture conversion via thinning can alleviate drought stress for P. strobus and that this species may be more sensitive to moisture stress when competition for water is high in denser stands. Evapotranspiration was 35 % lower in the silvopasture compared with the reference forest, primarily a result of lower transpiration and rainfall interception. While soil evaporation was greater in the silvopasture, this was not enough to offset the considerably lower transpiration and interception. We observed greater radial tree growth 1–3 years following conversion in the silvopasture as compared with the reference forest for T. canadensis and Q. rubra, but not for P. strobus. Overall, our results suggest that forest conversion to silvopasture (in lieu of clearcutting for new pasture) may mitigate the impacts of agricultural land use intensification and climate change on ecosystem services, especially in terms of sustaining hydrologic regulation functions. Further study is required to determine the generality of these results and whether these benefits extend beyond the first few years post-conversion.
Jilling A, Kane D, Williams A, Yannarell AC, Davis A, Jordan NR, Koide RT, Mortensen DA, Smith RG, Snapp SS, et al. Rapid and distinct responses of particulate and mineral-associated organic nitrogen to conservation tillage and cover crops. Geoderma. 2020;359 :114001.Abstract
Particulate organic matter (POM) is considered an “active” source of nitrogen (N) in cultivated soils, responding readily to management and being more physically accessible to decomposers than mineral-associated forms of organic matter. However, there is increasing evidence that mineral-associated organic matter (MAOM) can also exhibit short-term changes to management that may impact plant and microbial N dynamics. In this study, we investigated how N within soil organic matter fractions responded to three years of tillage and cover crop treatments. We collected soils from a row-crop (maize-soybean rotation) field experiment replicated across three sites in the north central and mid-Atlantic United States: a high-soil organic matter site (3.1% soil organic carbon) in Illinois (IL) and two sites in Michigan (MI) and Pennsylvania (PA) with lower soil organic matter content (1.0% and 1.4% soil organic carbon, respectively). Management treatments included two levels of tillage (chisel plow and ridge tillage) and two levels of cover crop (with and without rye cover crop). Using an optimized sonication method coupled with particle size separation, we isolated and analyzed for N content free POM, occluded POM, a coarse silt fraction, and MAOM. Using partial least squares regression, we also explored broad cross-site relationships between soil organic matter (SOM) fractions, soil N availability, and crop performance. Both particulate and fine fractions responded to tillage and cover crop treatments, but patterns varied by site and fraction. In the low-SOM MI and PA soils, ridge tillage and cover cropping both increased N within POM fractions. The response to ridge tillage was most pronounced, with a 76% and 24% increase in occluded POM N content in MI and PA, respectively. In contrast, at the IL site (high-SOM), the inclusion of cover crops led to higher N, specifically within the fine fractions (coarse silt and MAOM). Cover cropping increased MAOM N content in IL by 24%. When analyzing all sites together, variables associated with fine fractions were more closely associated with N mineralization and crop performance. MAOM can be responsive to short-term management practices and, along with POM, may also be potential sources of N for crops.
Wilhelm JA, Smith RG, Jolejole-Foreman MC, Hurley S. Resident and stakeholder perceptions of ecosystem services associated with agricultural landscapes in New Hampshire. Ecosystem Services. 2020;45.Abstract
Converting non-agricultural land to agricultural uses can result in trade-offs in ecosystem services. As provisioning services increase on new agricultural land, supporting, regulating, and cultural services may decrease. An improved understanding of how stakeholders value different land use types in terms of their perceived ecosystem services, as well as the relative visual appeal of different agricultural landscape features, could assist policymakers and land use planners in decision-making related to agricultural land use in New England, USA. We surveyed two survey samples in New Hampshire, food system stakeholders (e.g., farmers, public health professionals, and technical assistance providers) and the general population, to explore how perception of the visual appeal of specific farmland use types and importance of ecosystem services specifically related to agricultural land might differ between survey samples. Our objectives were to (1) explore how New Hampshire residents perceive the importance of seven ecosystem services, (2) evaluate how two groups of New Hampshire residents—the general public and those who indicated working with or in a food systems sector (food system stakeholders)—perceive these ecosystem services provided by specific agricultural landscapes and determine how those perceptions relate to the visual appeal of each landscape, and (3) assess how eight socio-economic factors may account for the differences between each survey sample in terms of their landscape perception and preference. Roughly 600 New Hampshire residents completed the survey, including 494 individuals from the public and 103 food system stakeholders. From a list of seven ecosystem services, clean water was ranked as the most important across both survey samples, with no significant difference between samples, while food production was ranked significantly higher by the food system stakeholders (p ≤ 0.001). Likewise, on a scale of most (4) to least (1) appealing, food system stakeholders ranked photorealistic visualizations of cropland higher than the public (p ≤ 0.001). Additionally, food system stakeholders ranked the appeal of forestland lower than the public (p = 0.007). Our findings suggest that there are differences in landscape preferences and perception of ecosystem service benefits between the general public and those who work with or in the food system. Future research is needed to determine how these differences in perception might affect land use planning and policymaking related to agricultural expansion and forestland preservation.
Menalled UD, Bybee-Finley KA, Smith RG, DiTommaso A, Pethybridge SJ, Ryan MR. Soil-Mediated Effects on Weed-Crop Competition: Elucidating the Role of Annual and Perennial Intercrop Diversity Legacies. Agronomy 2020, Vol. 10, Page 1373 [Internet]. 2020;10 (9) :1373. Publisher's VersionAbstract
Crop diversity may mediate the intensity of weed-crop competition by altering soil nutrient availability and plant-soil microbe interactions. A greenhouse experiment was conducted to analyze weed-crop competition in soils with varying crop diversity legacies. Soil greenhouse treatments included field soils (i.e., soil nutrient and microbial legacies), a sterile greenhouse potting mix inoculated with microorganisms of the field soils (i.e., microbial legacies), and a sterile greenhouse potting mix. Soils for the greenhouse experiment were sampled and assessed after two-years of conditioning with annual and perennial cropping systems under four levels of intercrop diversity. The greenhouse experiment involved growing one sorghum sudangrass (Sorghum bicolor (L.) Moench × S. sudanese Piper) crop plant and zero to six common lambsquarters (Chenopodium album L.) weed plants in soil from each diversity and cropping system treatment. The weed density treatments created a weed-crop competition gradient, which was used to quantify legacy effects of crop diversity. Weed-crop competition increased with crop diversity in both the field soil and inoculated soil treatments in the annual system. In the perennial system, differences in weed-crop competition intensity were driven by crop yield potential. In the perennial field soil treatment, crop yield potential was greatest in the highest diversity treatment, whereas in the perennial inoculated soil treatment, crop yield potential was greatest in the lowest diversity treatment. Results show potential for negative effects from previous crop diversity on weed-crop competition, and the divergent impact of microbial and nutrient legacies on crop yield potential. Future research should aim to evaluate the consistency of legacy effects and identify principles that can guide soil and crop management, especially in conservation agriculture where soil tillage and its microbial legacy reducing effects are minimized.
Orefice J, Smith RG, Carroll J, Asbjornsen H, Howard T. Forage productivity and profitability in newly-established open pasture, silvopasture, and thinned forest production systems. Agroforestry Systems. 2019;93 (1) :51–65.Abstract
There is growing interest among farmers in the northeast US in silvopasture and other practices that would help them expand their agricultural land base in this forested region. Unfortunately, little quantitative data exist regarding forage performance and economic outcomes associated with different forest-to-agriculture conversion strategies, particularly for the northern tier of states. This research examined forage dry matter production and quality in six forage treatments (orchardgrass, bluegrass, brome, and perennial ryegrass bicultures with white clover; cut hay; and a control) in newly-established silvopastures and open pastures converted from an early successional northern hardwood forest in New York. In addition, we conducted a financial analysis of the establishment of both agricultural systems, as well as a thinned forest treatment managed solely for wood products. Total forage dry matter production (planted forages plus volunteer grasses) was greater in open pastures than silvopastures in the first year after establishment; however, no differences in total forage production were found between silvopastures and open pastures in June or August of the second year. Total forage dry matter production was greater in the orchardgrass-white clover biculture compared to the control treatment in both years. Orchardgrass percent crude protein was lower in open pastures (10.7%) compared to silvopastures (12.9%) in June of year two. The financial analysis indicated that silvopasture outperformed open pasture and thinned forest treatments in terms of both IRR and NPV. We conclude that forage production in silvopastures can be competitive with that in open pastures on sites with a similar, forested, starting condition.
Stewart A, Coble A, Contosta AR, Orefice JN, Smith RG, Asbjornsen H. Forest conversion to silvopasture and open pasture: effects on soil hydraulic properties. Agroforestry Systems. 2019 :1–11.Abstract
{Growing demand for local products in the northeastern U.S. may incentivize forest conversion to pasture, degrading critical soil hydrologic properties such as surface infiltration (Kh) and subsurface saturated hydraulic conductivity (Ksat). Silvopasture, combining tree cover and grazing, may mitigate these impacts by maintaining the positive effects of trees on soil hydraulic properties. We tested this hypothesis using an experimental field manipulation to compare effects of forest conversion to open pasture versus silvopasture on Kh and Ksat at the Organic Dairy Research Farm (ODRF) and North Branch Farm (NBF). Measurements of surface Kh and Ksat at two soil depths (15 cm and 30 cm) were taken 1 and 4 years after treatment establishment at ODRF and NBF, respectively. Data were analyzed using a mixed effects modeling framework. Results show 15 cm Ksat was significantly lower in pasture compared to forest across both sites. However, in contrast to our hypothesis, soil hydraulic properties in silvopasture did not differ from other treatments at either site. Notwithstanding, silvopasture 15 cm Ksat at ODRF (9.4 cm h−1) was statistically similar to both the forest (22.6 cm h−1) and pasture (3.4 cm h−1) and exhibited a weak positive correlation with proximity to trees (R2 = 0.219
Schnecker J, Bowles T, Hobbie EA, Smith RG, Grandy AS. Substrate quality and concentration control decomposition and microbial strategies in a model soil system. Biogeochemistry. 2019;144 (1) :47–59.Abstract
Soil carbon models typically scale decomposition linearly with soil carbon (C) concentration, but this linear relationship has not been experimentally verified. Here we investigated the underlying biogeochemical mechanisms controlling the relationships between soil C concentration and decomposition rates. We incubated a soil/sand mixture with increasing amounts of finely ground plant residue in the laboratory at constant temperature and moisture for 63 days. The plant residues were rye (Secale cereale, C/N ratio of 23) and wheat straw (Triticum spp., C/N ratio of 109) at seven soil C concentrations ranging from 0.38 to 2.99%. We measured soil respiration, dissolved organic carbon (DOC) concentrations, microbial biomass, and potential enzyme activities over the course of the incubation. Rye, which had higher N and DOC contents, lost 6 to 8 times more C as CO2 compared to wheat residue. Under rye and wheat amendment, absolute C losses as CO2 (calculated per g dry soil) increased linearly with C concentration while relative C losses as CO2 (expressed as percent of initial C) increased with C concentration following a quadratic function. In low C concentration treatments (0.38–0.79% OC), DOC decreased gradually from day 3 to day 63, microbial C increased towards the end in the rye treatment or decreased only slightly with straw amendment, and microbes invested in general enzymes such as proteases and oxidative enzymes. At increasing C levels, enzyme activity shifted to degrading cellulose after 15 days and degrading microbial necromass (e.g. chitin) after 63 days. At the highest C concentrations (2.99% OC), microbial biomass peaked early in the incubation and remained high in the rye treatment and decreased only slightly in the wheat treatment. While wheat lost C as CO2 constantly at all C concentrations, respiration dynamics in the rye treatment strongly depended on C concentration. Our results indicate that litter quality and C concentration regulate enzyme activities, DOC concentrations, and microbial respiration. The potential for non-linear relationships between soil C concentration and decomposition may need to be considered in soil C models and soil C sequestration management approaches.
Ball MG, Caldwell BA, DiTommaso A, Drinkwater LE, Mohler CL, Smith RG, Ryan MR. Weed community structure and soybean yields in a long-term organic cropping systems experiment. Weed Science. 2019;67 (6) :673–681.Abstract
Weed management is a major challenge in organic crop production, and organic farms generally harbor larger weed populations and more diverse communities compared with conventional farms. However, little research has been conducted on the effects of different organic management practices on weed communities and crop yields. In 2014 and 2015, we measured weed community structure and soybean [Glycine max (L.) Merr.] yield in a long-term experiment that compared four organic cropping systems that differed in nutrient inputs, tillage, and weed management intensity: (1) high fertility (HF), (2) low fertility (LF), (3) enhanced weed management (EWM), and (4) reduced tillage (RT). In addition, we created weed-free subplots within each system to assess the impact of weeds on soybean yield. Weed density was greater in the LF and RT systems compared with the EWM system, but weed biomass did not differ among systems. Weed species richness was greater in the RT system compared with the EWM system, and weed community composition differed between RT and other systems. Our results show that differences in weed community structure were primarily related to differences in tillage intensity, rather than nutrient inputs. Soybean yield was lower in the EWM system compared with the HF and RT systems. When averaged across all four cropping systems and both years, soybean yield in weed-free subplots was 10% greater than soybean yield in the ambient weed subplots that received standard management practices for the systems in which they were located. Although weed competition limited soybean yield across all systems, the EWM system, which had the lowest weed density, also had the lowest soybean yield. Future research should aim to overcome such trade-offs between weed control and yield potential, while conserving weed species richness and the ecosystem services associated with increased weed diversity.
Wilhelm JA, Smith RG. Ecosystem services and land sparing potential of urban and peri-urban agriculture: A review. Renewable Agriculture and Food Systems. 2018;33 (5) :481–494.Abstract
Agricultural expansion contributes to the degradation of biodiverse ecosystems and the services these systems provide. Expansion of urban and peri-urban agriculture (UPA), on the other hand, may hold promise to both expand the portfolio of ecosystem services (ES) available in built environments, where ES are typically low and to reduce pressure to convert sensitive non-urban, non-agricultural ecosystems to agriculture. However, few data are available to support these hypotheses. Here we review and summarize the research conducted on UPA from 320 peer-reviewed papers published between 2000 and 2014. Specifically, we explored the availability of data regarding UPA's impact on ES and disservices. We also assessed the literature for evidence that UPA can contribute to land sparing. We find that the growth in UPA research over this time period points to the emerging recognition of the potential role that UPA systems play in food production worldwide. However, few studies (n = 15) place UPA in the context of ES, and no studies in our review explicitly quantify the land sparing potential of UPA. Additionally, while few studies (n = 19) quantify production potential of UPA, data that are necessary to accurately quantify the role these systems can play in land sparing, our rough estimates suggest that agricultural extensification into the world's urban environments via UPA could spare an area approximately twice the size of the US state of Massachusetts. Expanding future UPA research to include quantification of ES and functions would shed light on the ecological tradeoffs associated with agricultural production in the built environment. As food demand increases and urban populations continue to grow, it will be critical to better understand the role urban environments can play in global agricultural production and ecosystem preservation.
Smith RG, Birthisel SK, Bosworth SC, Brown B, Davis TM, Gallandt ER, Hazelrigg A, Venturini E, Warren ND. Environmental Correlates with Germinable Weed Seedbanks on Organic Farms across Northern New England. Weed Science. 2018;66 (1).Abstract
© 2017 Weed Science Society of America. The northern New England region includes the states of Vermont, New Hampshire, and Maine and encompasses a large degree of climate and edaphic variation across a relatively small spatial area, making it ideal for studying climate change impacts on agricultural weed communities. We sampled weed seedbanks and measured soil physical and chemical characteristics on 77 organic farms across the region and analyzed the relationships between weed community parameters and select geographic, climatic, and edaphic variables using multivariate procedures. Temperature-related variables (latitude, longitude, mean maximum and minimum temperature) were the strongest and most consistent correlates with weed seedbank composition. Edaphic variables were, for the most part, relatively weaker and inconsistent correlates with weed seedbanks. Our analyses also indicate that a number of agriculturally important weed species are associated with specific U.S. Department of Agriculture plant hardiness zones, implying that future changes in climate factors that result in geographic shifts in these zones will likely be accompanied by changes in the composition of weed communities and therefore new management challenges for farmers.
Atwood LW, Mortensen DA, Koide RT, Smith RG. Evidence for multi-trophic effects of pesticide seed treatments on non-targeted soil fauna. Soil Biology and Biochemistry. 2018;125 :144–155.Abstract
The use of pesticide seed treatments containing neonicotinoid insecticides is widespread in large-scale row crop agriculture. Recently, use of pesticide seed treatments has come under scrutiny due to concerns over non-target impacts on beneficial insects (e.g., honey bees) and the environment. Amidst these growing concerns, however, few studies have examined how pesticide seed treatments may impact soil faunal communities across multiple feeding guilds and the soil processes they regulate. We grew corn and soybean with and without pesticide seed treatment for three years and measured the response of the soil faunal detritivore, herbivore, mixed, and predator feeding guilds, nitrogen mineralization, and surface litter decomposition at three time points each year. We found the effects of seed treatment on the soil faunal community varied in direction and magnitude by year and feeding guild and were most apparent in the predator and detritivore guilds. Guild-level effects tended to be strongest soon after planting but remained apparent throughout the crop growing season, particularly in the predator and mixed feeding guilds. We found no evidence that pesticide seed treatment affected the herbivore guild—the intended target of the seed treatment, or nitrogen mineralization, surface litter decomposition, or grain yields. Collectively, these data suggest that pesticide seed treatments can alter the abundance, richness, and diversity of all non-targeted soil faunal guilds. Additional research will be necessary to determine the longer-term significance of pesticide seed treatment-driven changes in non-target soil faunal communities in agroecosystems.
Lounsbury NP, Warren ND, Wolfe SD, Smith RG. Investigating tarps to facilitate organic no-till cabbage production with high-residue cover crops. Renewable Agriculture and Food Systems. 2018.Abstract
© Cambridge University Press 2018. High-residue cover crops can facilitate organic no-till vegetable production when cover crop biomass production is sufficient to suppress weeds (>8000 kg ha1), and cash crop growth is not limited by soil temperature, nutrient availability, or cover crop regrowth. In cool climates, however, both cover crop biomass production and soil temperature can be limiting for organic no-till. In addition, successful termination of cover crops can be a challenge, particularly when cover crops are grown as mixtures. We tested whether reusable plastic tarps, an increasingly popular tool for small-scale vegetable farmers, could be used to augment organic no-till cover crop termination and weed suppression. We no-till transplanted cabbage into a winter rye (Secale cereale L.)-hairy vetch (Vicia villosa Roth) cover crop mulch that was terminated with either a roller-crimper alone or a roller-crimper plus black or clear tarps. Tarps were applied for durations of 2, 4 and 5 weeks. Across tarp durations, black tarps increased the mean cabbage head weight by 58% compared with the no tarp treatment. This was likely due to a combination of improved weed suppression and nutrient availability. Although soil nutrients and biological activity were not directly measured, remaining cover crop mulch in the black tarp treatments was reduced by more than 1100 kg ha1when tarps were removed compared with clear and no tarp treatments. We interpret this as an indirect measurement of biological activity perhaps accelerated by lower daily soil temperature fluctuations and more constant volumetric water content under black tarps. The edges of both tarp types were held down, rather than buried, but moisture losses from the clear tarps were greater and this may have affected the efficacy of clear tarps. Plastic tarps effectively killed the vetch cover crop, whereas it readily regrew in the crimped but uncovered plots. However, emergence of large and smooth crabgrass (Digitaria spp.) appeared to be enhanced in the clear tarp treatment. Although this experiment was limited to a single site-year in New Hampshire, it shows that use of black tarps can overcome some of the obstacles to implementing cover crop-based no-till vegetable productions in northern climates.
Whalen ED, Smith RG, Grandy AS, Frey SD. Manganese limitation as a mechanism for reduced decomposition in soils under atmospheric nitrogen deposition. Soil Biology and Biochemistry. 2018;127 :252–263.Abstract
Long-term atmospheric nitrogen (N) deposition has been shown to reduce leaf litter and lignin decomposition in temperate forest soils, leading to an accumulation of soil carbon (C). Reduced decomposition has been accompanied by altered structure and function of fungal communities, the primary decomposers in forest ecosystems; however, a mechanistic understanding of fungal responses to chronic N enrichment is lacking. A reduction in soil and litter manganese (Mn) concentrations under N enrichment (i.e., Mn limitation) may help explain these observations, because Mn is a cofactor and regulator of lignin-decay enzymes produced by fungi. We conducted a laboratory study to evaluate the effect of Mn availability on decomposition dynamics in chronically N-enriched soils. We measured litter mass loss, lignin relative abundance, and lignin-decay enzyme activities, and characterized the litter fungal community by ITS2 metabarcoding. We observed a significant positive correlation between Mn availability and lignin-decay enzyme activities. In addition, long-term (28 years) N enrichment increased the relative abundance of ‘weak' decomposers (e.g., yeasts), but this response was reversed with Mn amendment, suggesting that higher Mn availability may promote fungal communities better adapted to decompose lignin. We conclude that Mn limitation may represent a mechanism to explain shifts in fungal communities, reduced litter decomposition, and increased soil C accumulation under long-term atmospheric N deposition.
Jilling A, Keiluweit M, Contosta AR, Frey S, Schimel J, Schnecker J, Smith RG, Tiemann L, Grandy AS. Minerals in the rhizosphere: overlooked mediators of soil nitrogen availability to plants and microbes. Biogeochemistry. 2018;139 (2) :103–122.Abstract
Despite decades of research progress, ecologists are still debating which pools and fluxes provide nitrogen (N) to plants and soil microbes across different ecosystems. Depolymerization of soil organic N is recognized as the rate-limiting step in the production of bioavailable N, and it is generally assumed that detrital N is the main source. However, in many mineral soils, detrital polymers constitute a minor fraction of total soil organic N. The majority of organic N is associated with clay-sized particles where physicochemical interactions may limit the accessibility of N-containing compounds. Although mineral-associated organic matter (MAOM) has historically been considered a critical, but relatively passive, reservoir of soil N, a growing body of research now points to the dynamic nature of mineral-organic associations and their potential for destabilization. Here we synthesize evidence from biogeoscience and soil ecology to demonstrate how MAOM is an important, yet overlooked, mediator of bioavailable N, especially in the rhizosphere. We highlight several biochemical strategies that enable plants and microbes to disrupt mineral-organic interactions and access MAOM. In particular, root-deposited low-molecular-weight exudates may enhance the mobilization and solubilization of MAOM, increasing its bioavailability. However, the competitive balance between the possible fates of N monomers—bound to mineral surfaces versus dissolved and available for assimilation—will depend on the specific interaction between mineral properties, soil solution, mineral-bound organic matter, and microbes. Building off our emerging understanding of MAOM as a source of bioavailable N, we propose a revision of the Schimel and Bennett (Ecology 85:591–602, 2004) model (which emphasizes N depolymerization), by incorporating MAOM as a potential proximal mediator of bioavailable N.