In the state of Maryland, the adoption of winter cover crops is very high due to the Maryland Cover Crop Program (MCCP), which provides farmers with incentives to help offset cover crop expenses. The main goal of the MCCP is to enhance soil N scavenging and reduce nitrate (NO3–) loading into the Chesapeake Bay; the impact of cover crops on N cycling in the subsequent cash crop is not emphasized by the program. Farmers are aware that grass cover crops increase N fertilizer requirements due to N immobilization. However, farmers may not know how much N is being released or immobilized by their cover crops and end up applying more or less fertilizer than required by the succeeding main crop. Over-application undermines the goals of MCCP, while under-application may reduce yields and discourage future cover crop use. Thus, it is critical to adjust N fertilizer application based on N availability from cover crop residue decomposition to optimize N use efficiency and corn (Zea mays L.) performance.
The large variability in climate between years and among farms makes it difficult to accurately assess soil N availability from cover crop surface residues. Climate has a strong effect on residue decomposition, with decay rates accelerating proportionately with increasing moisture and temperature. Previous cover crop decomposition studies in the region were used to validate an existing N management decision support tool. It was observed that the existing tool often do not accurately predict soil N availability from surface residues. Model performance has been influenced by fluctuations in moisture and temperature at the soil surface. The soil environment has much lower fluctuations in moisture and temperature, key factors in driving decomposition. This projects aims to understand N mineralization kinetics from surface-applied cover crop residues in controlled laboratory conditions and improve the performance of existing decision support tool to accurately predict N availability from surface-applied cover crop residues in no-till corn systems.
Using data from on-farm and controlled laboratory experiments, this project aims to extend the applicability of the existing decision support tool to better manage N in cover crop based no-till corn systems. Finally, this project will likely contribute to the increased adoption of cover crops in the mid-Atlantic US as more farmers and agricultural professionals become proficient in the use of this new decision support tool. Therefore, findings from this project not only impact the performance of cover crop-based no-till corn systems at the farm level, but also support the goals of the MCCP and NE-SARE through increased cover crop adoption at the state and regional levels, respectively.
The specific objectives of the proposed project are to:
(1) Assess the ability of cover crops to scavenge soil N using meta-analytic approach.
(2) Determine the interactive effect of moisture and temperature on C and N mineralization from surface-applied cover crop residues. I hypothesize that soil N availability from surface-applied cover crop residue decomposition will increase with increasing soil moisture and temperature.
(3) Determine the effect of dry-wet cycles and residue placement on C and N mineralization.
(4) Investigate the factors controlling decomposition and concomitantly, N release from surface-mulched cover crop residues in no-till corn systems.
Winter cover crops play a critical role in nitrogen (N) management, both as N scavenger while growing and as N supplier to the succeeding main crop while decomposing. Many farmers believe non-legume cover crops immobilize soil N and make it less available to the main crop, while legume cover crops supply N to the main crop during decomposition. However, they often do not know how much N is being immobilized or released by their cover crops. Farmers, therefore, find it very difficult to manage N for the succeeding main crop. Although few decision support tools such as Cover Crop Nitrogen Calculator are available, they often do not accurately predict soil N availability from surface-mulched residues because moisture and temperature greatly fluctuates in surface residues. In this project, I propose to assess the performance of model in predicting decomposition and subsequently, N release from surface residues using controlled laboratory and field studies. This research will be conducted on 35-40 farms across mid-Atlantic US and laid out in strip corn plots with and without cover crops. Using the on-farm data, I will investigate the factors controlling cover crop decomposition. By the end of this project, farmers will benefit from an improved decision support tool, which they can use to adjust N fertilizer requirements and mange N effectively in cover crop based no-till corn systems. This will lead to increased adoption of cover crops and improved agricultural sustainability.
I performed meta-analysis, controlled laboratory and on-farm experiments to address the objectives of this project. The objective 1 of this project used meta-analytic approach. The details of the methodology and the findings of the meta-analysis were published in a peer-reviewed journal. In brief, a literature search was performed in ISI Web of Science database to find the articles that compared nitrate leaching between cover crop and no cover crop treatments. In total, we found 28 relevant articles that were finally included. Data were extracted and compiled in excel sheet. The multi-level mixed effects meta-analytic model was performed to investigate the effects of cover crops on nitrate leaching using the nlme package in R. The effect size of cover crops on nitrate leaching was calculated as the natural logarithm of the response ratios and the results of the analysis were back-transformed for ease of interpretation. We also performed moderator analysis to investigate the effects of co-variates (non-legume category, planting dates, shoot biomass, and relative precipitation) on the mean effect size of non-legumes on reducing nitrate leaching.
To address objective 2 and 3, we conducted two separate controlled laboratory experiments in a customized air-flow system designed within a CONVIRON E7/2 growth chamber. The first experiment was conducted to determine the interactive effects of moisture (ψ : -10.0, -5.0, -1.5, and -0.03 MPa) and temperature (15, 25, and 35 °C) on C and N mineralization from surface-applied cover crop residues. Surface soil (0-10 cm) samples were collected from a commercially managed corn field at the Beltsville Agricultural Experiment Station in Beltsville, MD, USA. Field moist soil was sieved through 4-mm sieve, the roots were manually removed, and the soil was homogenized. The soil was then air-dried, ground and passed through a 2-mm sieve, then stored at room temperature until the start of the experiment. Cover crop residues used in this experiment consisted of two cover crop species harvested at different growth stages: (a) Late-killed crimson clover (flowering stage), (b) Late-killed cereal rye (flowering stage), and (c) Early-killed cereal rye (tillering stage). Soil and residues with specific water potential (ψ) were prepared. To prepare microcosms, all soils (50 g oven-dried equivalent) were packed in 100 mL plastic beakers (5.0 cm i.d.). Cover crop residues with a given ψ were surface-applied at a rate of 3000 kg ha-1 on soils with the same ψ. Cover crop resides were composed of both leaves and stems mixed in a definite proportion. An unamended soil as control was left for each moisture-temperature combination.
Headspace gas sampling was collected throughout the 150-d incubation period from microcosms enclosed in mason jars to determine CO2 and N2O emission rates from cover crop residues. Gas samples were drawn from each jar and transferred into 12-mL N2-flushed glass vials (Labco Ltd., Lampeter, UK) fitted with butyl rubber septa. The samples were analyzed within a week of collection using a Varian GC450 gas chromatograph (Agilent Technologies, Santa Clara, CA) equipped with thermal conductivity detector for CO2 and an electron capture detector for N2O.
The microcosms were destructively sampled at 15, 30, 60, 100, and 150-d after the initiation of experiment and analyzed for total inorganic N (NH4+ + NO3–) contents. The microcosms were extracted with 500 mL of 1 mol L-1 KCl (at a 1:10 soil:extractant ratio) after overnight shaking in 1-L plastic bottles. The KCl extracts were filtered through Whatman No. 42 filter paper and a subsample of filtered extracts were stored at -18 °C until analysis. The filtered extracts were analyzed colorimetrically for NH4+ and NO3– contents using a SEAL AutoAnalyzer 3 High Resolution (SEAL Analytical Inc., Mequon, WI).
Data analysis for objective 2: The C and N mineralization data over time was fitted to first-order rate kinetic model using nlme package in R to determine potentially mineralizable C or N and rate constants. Treatment means were compared by two-way analysis of variance performed separately for each cover crop residue using agricolae package in R. Significant differences among treatment means were determined using least significant difference (LSD) tests at α value of 0.05. To model the effects of T and ψ on net C or N mineralization from surface-applied cover crop residues, we calculated residue moisture-temperature factor (RMTFAC) for each residue separately. The RMTFAC at each T and ψ combination was calculated by dividing the net C or N mineralized in 150-d by the maximum value. For all cover crop residues, the maximum value of net C or N mineralized was obtained under optimum conditions (-0.03 MPa and 35 °C). Therefore, RMTFAC could be defined as the relative amounts of net C or N mineralized from cover crop residues when compared at optimum conditions. The RMTFAC ranged between 0 to 1 and allowed comparison across residue types. The RMTFAC were exponentially fitted to ψ at different T using nls function in R. The empirical constants of the exponential models were related to T using lm function in R.
Data analysis for objective 3: This experiment is on-going. Headspace gas samples and soil N extractions will be analyzed. Microbial community in the residue and soil samples will be determined to fill the missing knowledge gap that we have with regard to the role of microbes on decomposing surface residues.
Objective 4 of this project will be addressed by conducting multiple on-farm experiments across the mid-Atlantic US. Experiments will be conducted in separate fields each year to demonstrate the short-term benefits of cover crops on soil N scavenging and soil N availability in no-till corn systems. The experiment will be established as a strip-plot design with and without cover crops. There will be two sub-plots within each strip-plot. Data on cover crop biomass, residue quality, soil N scavenging, soil N availability from residue decomposition, and corn yields will be collected.
Farmers and extension agents will be directly involved in this project. With the help of county extension agents, I will contact the farmer-collaborators with a project description and request for their participation. Farmers will manage both cover crops and the subsequent corn crop. Farmers will make all farm management decisions including cover crop species, planting and termination methods/timings, fertilization, and irrigation. Growers will be required to either not plant a cover crop in the no cover crop control strips or spray with a herbicide ~10 days after planting. Since we will not dictate farmers’ management, this experiment will permit us to examine the short-term benefits of cover crops under a wide range of soil types, climatic conditions, and management practices. If required, assistance will be provided to farmers in developing the no cover crop strips (i.e., transport UTV mounted herbicide sprayer for cover crop termination).
Data Collection: At all farm sites, cover crop biomass will be sampled from two separate m2 quadrants in each subplot in the spring prior to termination. The fresh weight of cover crop biomass will be recorded and used to construct cover crop litter bags to assess their decomposition kinetics. A subset of this sample will be weighed fresh and then oven-dried at 60°C and dry weight recorded. The biomass samples will be mechanically ground and analyzed for C and N concentrations using CN elemental analyzer (Leco TruMac CN, St. Joseph, MI). A subsample will be sent to the University of Georgia analytical laboratory to determine the residue quality (cellulose, hemi-cellulose, and lignin content) using near-infrared spectroscopy (NIRS).
Using residue litter bag studies, the proportion of cover crop biomass and N remaining in the litter bags will be tracked over time. The dimension of each litter bag is 60 by 26 cm. I will prepare residue litter bags using the fresh biomass sampled from each subplot. There will be two sets of six litter bags to examine decomposition kinetics of cover crop surface residues in each subplot. Fresh weight of cover crop biomass will be evenly distributed within the litter bags. Out of six bags, one bag will be sampled on the same day as termination. The remaining five bags will be deployed on the soil surface in between corn rows using landscape staples. The leftover litter bags will be retrieved over time after 2, 2, 3, 4, and 4 weeks of termination to determine mass loss on an ash-free basis. Upon retrieval, the litter bags along with biomass will be oven-dried at 60°C for two weeks, and the dry weight recorded. The oven-dried biomass will then be ground and a subsample will be analyzed for C and N concentrations using a CN elemental analyzer (Leco TruMac CN, St. Joseph, MI). Data on cover crop biomass and C and N content will be used to fit cover crop decay and N release curves for each site.
Data analysis: The residue litter bag decomposition data was fitted to first-order rate kinetic model to determine the decay rate (k) values. In the preliminary analysis, the k values were linearly related to biochemical constituents of the residues using lm function in R. Going forward,we will also perform multiple linear regression and structural equation modelling to determine the direct and indirect effects of various factors (latitude, quantity, quality, and climate) on k values. Data from on-farm experiments will also be used to evaluate the performance of modified CERES-N mulch model in predicting decomposition and N release from surface residues.
The meta-analysis (objective 1) is already published in a peer-reviewed journal (Title: Cover crops reduce nitrate leaching in agroecosystems: a global meta-analysis). We found that non-legumes consistently reduced nitrate leaching by 56% compared to no cover crop controls. Legumes and mixtures, however, can reduce nitrate leaching during its growth, but release N quickly following their termination as a result of low C/N ratio in their residues. The effect of non-legumes in reducing nitrate leaching was less impacted by soil texture, soil organic C, and soil tillage. The most influencing variables on the effectiveness of non-legumes in reducing nitrate leaching were cover crop planting dates, shoot biomass, and relative precipitation (precipitation relative to the long-term normal precipitation for a particular site). I found that delaying cover crop planting dates reduced their ability to reduce nitrate leaching, probably because of the shortened growth period and concomitantly decreased biomass (shoot and root) yields and N uptake as compared to early-planted cover crops. Supporting this hypothesis, I found that the efficacy of non-legumes in reducing nitrate leaching was positively correlated with the shoot biomass at termination. The quadratic curve provided the best fit and the peak in nitrate leaching was observed at biomass levels above 2 Mg ha-1. I further found that the effectiveness of non-legumes in reducing nitrate leaching decreased with increasing relative precipitation suggesting greater efficacy in dry years as compared to wet years. Overall, this meta-analysis confirms many prior studies showing that non-legumes are an effective means to reduce nitrate leaching and should be integrated into cropping systems to improve water quality.
Besides nitrate leaching reductions, cover crops can provide N benefits to the subsequent cash crop following termination. The controlled laboratory experiment suggest that the C and N mineralization from surface-applied cover crop residues was regulated by the interactive effects of residue moisture (ψ) and temperature (T) (objective 2). The C and N mineralization data over time was adequately described by the first-order rate kinetic model at all moisture and temperature combinations. The net C and N mineralized in 150-d increased with increase in ψ or T or both. This was probably due to greater enzymatic breakdown of polymers at higher T, greater solubility and diffusivity of substrate at higher ψ, and hence, greater substrate availability for microbial decomposition. Moreover, the microbial activity was enhanced at higher ψ. Substantial amount of decomposition of surface residues at relatively dry conditions (ψ = -10.0 MPa) suggests that fungi may be the primary decomposers of surface residues as they have very low water stress thresholds. The ability of fungi to form extensive hyphal networks allow them to exploit resources (water and nutrients) from both soil and surface residues and hence, decomposition even at very dry conditions. There are several reports supporting this hypothesis, some of which even conclude that fungal translocation of soil N to surface residues may be the primary mechanism of N immobilization during the decomposition of high C/N ratio residues left on the surface. In the current study, net N immobilization was found with late-killed rye residues with C/N ratio of 88.
I found that the RMTFAC for all residues were exponentially related to ψ at all temperatures studied. The empirical constants of the exponential model were linearly related to the inverse of T, suggesting enhanced effect of ψ on C and N mineralization from surface residues with increasing T. Moreover, the empirical constants of the model differed between early and late-killed residues for C mineralization, but not for N mineralization. Therefore, the response surface of RMTFAC for C mineralization was calculated separately for early and late-killed residues. The modified response surface curves will be integrated into the modified CERES-N mulch model and the model will be calibrated using on-farm data.
Objective 4: Agroecosystem services from cover crops depend on the rate at which cover crops decompose (k values). The first-order k value is a function of both extrinsic (management: cover crop species, cover crop quantity and quality at termination) and intrinsic (soil: soil microbial community and diversity) factors and climate: moisture and temperature). Using the data from multi-state on-farm experiments conducted during 2017-18, we investigated the relationship between k values and various controlling factors. We found that the k value increased with increasing N and carbohydrate concentrations in the residues. The k value decreased with increasing C/N ratio, and also with the increase in cellulose, hemi-cellulose, and lignin concentrations in the residues. Combining multiple quality parameters, such as (cellulose + hemi-cellulose)/N) and lignin/N, increased R2. We hypothesize that including climate variables such as rainfall and temperature might further improve prediction of k values.
Education & Outreach Activities and Participation Summary
During 2018 and 2019, I conducted a workshop/ field day in Kent county UMD extension office, Eastern MD. Around 40 farmers/year participated in the workshop where I laid out the objectives of this on-farm research. I demonstrated the methodology of biomass sampling, soil core sampling, cover crop decomposition assay using nylon litter bag, and soil water monitoring using the custom-developed soil water sensors. Many farmers in the workshop were already involved in our research. Some of the farmers who were new to us showed great interest to be the part of our project. They were highly interested in our project. We are going to facilitate our on-farm effort in their farms in this upcoming year.
Two field days were conducted so far to talk about our on-farm research efforts. Both field days were conducted at the USDA Beltsville Agricultural Research Center (BARC) research farms at Beltsville, MD. The field days were conducted to the organic and cover crop societies during the 2018 ASA-SSSA-CSA tri-society meeting and 2019 Northeast Cover Crop Council (NECCC) regional meeting. During this on-farm demonstration, I got an opportunity to demonstrate the use of litter bag to assess cover crop decomposition, and the use of custom-developed soil water sensor technology for monitoring the soil water in systems with and without cover crops. Each time, we had more than 100 participants; including farmers, researchers, ag. professionals, and extension agents.
I have presented my research findings to my peers, research scientists and farmers through regional and national conferences. Below are the topic of my talks.
- Thapa R., S. Mirsky, K.L. Tully, M.L. Cabrera, H.H. Schomberg, C. Reberg-horton, J.W. Gaskin, D. Timlin, E. Sweep, B. Davis, S. Seehvaer, E.H. Henriquez Inoa, and A. Poncet. 2019 ASA-CSSA-SSSA International Annual Meeting, San Antonio, Texas: Nov 10-13, 2019. Title: Cover crop decomposition in no-till corn fields: controlling factors. (poster)
- Thapa R., S. Mirsky, K.L. Tully, M.L. Cabrera, C. Dann, H.H. Schomberg, D. Timlin, and C. Reberg-horton, 2019 ASA-CSSA-SSSA International Annual Meeting, San Antonio, Texas: Nov 10-13, 2019. Title: Effect of temperature and residue water potential on C and N mineralization from surface-applied cover crop residues. (poster)
- Thapa R., S. Mirsky, K.L. Tully, M.L. Cabrera, H.H. Schomberg, C. Reberg-horton, J.W. Gaskin, D. Timlin, E. Sweep, B. Davis, S. Seehvaer, E.H. Henriquez Inoa, and A. Poncet. 2019 Northeast Cover Crop Council (NECCC) Annual Meeting, College Park, MD: Nov: 7-8, 2019. Title: Cover crop decomposition in no-till corn fields: controlling factors. (poster)
- Thapa R., S. Mirsky, K.L. Tully, M.L. Cabrera, C. Dann, H.H. Schomberg, D. Timlin, and C. Reberg-horton, 2019 Northeast Cover Crop Council (NECCC) Annual Meeting, College Park, MD: Nov: 7-8, 2019. Title: Effect of temperature and residue water potential on C and N mineralization from surface-applied cover crop residues. (poster)
- Thapa R., S.B. Mirsky, K.L. Tully, H. Schomberg, S.C. Reberg-Horton, J.W. Gaskin, M. Cabrera, E. Sweep, and N. Richards. 2018 Northeast Cover Crops Council (NECCC) annual meeting, State College, PA: Nov 15-16, 2018. Title: Cover crop decomposition in no-till corn fields: controlling factors. (poster)
- Thapa R., S.B. Mirsky, and K.L. Tully. 2018 ASA and CSSA annual meeting, Baltimore, MD: Nov 4-7, 2018. Title: Cover crops reduce nitrate leaching in agroecosystems: a global meta-analysis. (oral)
- Thapa R., S.B. Mirsky, K.L. Tully, H. Schomberg, S.C. Reberg-Horton, J.W. Gaskin, and M. Cabrera. 2018 ASA and CSSA annual meeting, Baltimore, MD: Nov 4-7, 2018. Title: Cover crop decomposition in no-till corn fields: controlling factors. (oral)
- Thapa R., S.B. Mirsky, and K.L. Tully. 2017 Northeast Cover Crops Council (NECCC) annual meeting, Ithaca, NY: Nov. 9, 2017. Title: Cover crops and nitrate leaching. (poster)
- Thapa R., H. H. Poffenbarger, K.L. Tully, V. Ackyord, M. Kramer, and S.B. Mirsky. 2017 ASA-CSSA-SSSA annual meeting, Tampa, FL: Oct 24, 2017. Title: Biomass production and nitrogen accumulation by hairy vetch-cereal rye mixtures: a meta-analysis. (oral)
I also gave two guest lectures to students in UMD-Plant Sciences department to highlight the role cover crops play on improving agricultural sustainability. During this lectures, I presented some of the findings from this project.
- Thapa R. Nov 5, 2019. Title: Cover crops for agricultural sustainability. (Invited guest lecture in ‘PLSC 303 – Global Food Systems’ class at UMD-PSLA).
- Thapa R. Nov 5, 2018. Title: Managing soil health with cover crops. (Invited guest lecture in ‘PLSC 405 – Agroecology’ class at UMD-PSLA).
I have also published two meta-analysis articles in peer-reviewed journals. The articles were:
- Thapa, R., H. Poffenbarger, K. Tully, V.J. Ackroyd, M. Kramer, and S.B. Mirsky. 2018. Biomass production and nitrogen accumulation by hairy vetch-cereal rye mixtures: a meta-analysis. Agronomy Journal. 110(4): 1197-1208.
- Thapa, R., S.B. Mirsky, and K. Tully. 2018. Winter cover crops reduced nitrate leaching in agro-ecosystems: a meta-analysis. Journal of Environmental Quality. 47(6): 1400-1411.