Final report for GNE22-301
Project Information
Many Northeast farmers plant cover crops (CC) to build soil health, provide benefits to subsequent crops, and mitigate the negative environmental impacts of agriculture. Although the long-term benefits of CC are well established, additional research is needed to examine short-term, species-mediated impacts of CC on soil health and nutrient cycling. These species-mediated influences on soil health parameters must be better understood to translate soil health concepts into the development of soil health best management practices. Decomposing CC residues impact nutrient turnover in soils by directly altering active carbon (C), which is the portion of soil C that is immediately available as a microbial food source; active C serves as fuel for the microbial soil engine. This project characterized the different rates of decomposition, measured by C and nitrogen (N) release, of aerial and root residues of winter-killed oat, daikon radish, and field pea CC (monocultures and mixed plantings) as they decomposed from April-July and quantified changes to active soil C in each treatment during the decomposition process. This project served as a supplement to a larger study and contributed to a comprehensive assessment of the relationships between active soil C, microbial respiration, soil moisture, and soil food web functioning, and the associated implications for soil health and nutrient cycling. Overall, the decomposing residues of annual, fall planted cover crops consistently contributed significant amounts of C (up to 800 kg ha-1) to agricultural systems in the spring, while the N release from cover crop residues varied from year to year. While daikon radish, field pea, and mixed cover crops released most of the N found in their residues by the beginning of June (65-91%), nutrient release from oat residues was more gradual and continued past the spring. Due to the annual variability in N release rate, it is inadvisable that the residues of annual, fall planted CC be used as a reliable source of supplemental N when generating nutrient management plans. CC species did not significantly impact active C during spring residue decomposition in this study, suggesting that two consecutive years of planting annual, fall cover crops is likely not enough time to detect differences in active C. Additionally, the relationships between active C and microbial respiration, two commonly used indicators of soil health and nematode-based indices of soil food web condition were weak, indicating that these soil health indicators cannot be used to infer soil food web status.
Objective 1 – Classify the C and N release of three annual, fall-planted CC common in the Northeast (oat, field pea, forage radish) and their mixture during spring residue decomposition
Objective 2 – Quantify changes in soil active C during spring residue decomposition of the three CC species, representing different C:N compositions, compared to a mixture of all three species over the course of two years
Objective 3 – Characterize the relationships between cover crop residue decomposition, active soil C, microbial respiration, soil food web status, and soil moisture over the course of two years
The goals of this project were to 1) develop a C and N release profile from the residues of three fall-planted, annual cover crop species (oat, forage radish, field pea) and their mixture during decomposition, and 2) classify the species-mediated impact of CC decomposition on active soil carbon throughout the season.
CC contribute to agricultural sustainability by building soil health, naturally enhancing soil fertility, minimizing environmental pollution, and reducing reliance on off-farm inputs. Although the benefits of CC are well established, additional research is required to distinguish between seasonal, annual, and long-term impacts of CC use on specific soil health indicators, including active C. The long-term effects of CC, such as reduced soil compaction, increased soil organic matter (SOM), and improved aggregate stability have been well documented. However, it is understood that these benefits are not realized in the short term. Presently, both in-field and laboratory analyses of these metrics are unable to capture short-term changes to soil health. Understanding the seasonal (3-4 months) and short-term (1-2 years) impacts of CC on soil health and nutrient cycling can guide on-farm CC selection to inform both short-term nutrient management plans and long-term soil health goals.
CC species chosen for this project vary in composition, function, and biomass production. Oat is a fast-growing cereal grain with a high C:N ratio and fibrous roots that scavenge for residual soil nutrients (such as nitrate). Field pea (a legume) and daikon radish (a brassica) have low C:N ratios, and thus decompose more rapidly. The taproot of forage radish can access nutrients deep in the soil profile, while field pea can supplement successor crops with biologically fixed atmospheric N. Mixed CC plantings balance the composite C:N ratio, resulting in gradual decomposition and nutrient release. Currently, research into CC residue decomposition dismisses roots as providing residual decomposition contributions. However, the biomass, composition, and nutrient content of plant roots differ from that of the aerial parts.
Both living CC roots and decomposing CC residue biomass impact soil microbial communities and ecosystem function via C and N inputs. In the Northeast, annual CC planted in late summer provide a food source for soil microbes in fall (via living root exudates) and early spring (via C and N released from residue decomposition). The fraction of SOM that is readily available for microbial consumption, or active C, is especially sensitive to management and nutrient inputs, providing a metric to measure short-term soil health changes. Because active C drives energy into the soil food web, influencing the rate of nutrient turnover, it is recommended as a soil health indicator by the USDA-NRCS. This project classified the species-mediated impacts of oat, forage radish, and pea residues (planted in monocultures and as a mixed crop) on active soil C during decomposition in the spring during the first two years of CC planting.
Research
This project was a highly valuable supplement to a larger research program funded by the Massachusetts Department of Agricultural Resources (MDAR). The focus of the larger research project was to enhance the resiliency and sustainability of garlic production in Massachusetts by evaluating the benefits of an alternative garlic production system in which garlic is relay-cropped into standing cover crop residues. Table 1 displays the data that was collected for each separate project and how it was combined to generate new results.
The goal of this research project was to provide a comprehensive analysis of the annual, species-mediated impacts of spring cover crop decomposition on soil health and nutrient cycling.
Two field experiments were conducted at the UMass Crop, Animal, Research and Education Center in South Deerfield, MA and repeated for two years (2020-2021; 2021-2022). Plant tissue and soil samples collected from these experiments were used to complete laboratory analyses and generate data for the project described in this proposal.
Table 1. Description of the data collection for this project and the larger, pre-existing research initiative (MDAR project), along with an explanation of how the data will be synthesized to generate new results.
|
Sample Type |
Data collected for MDAR project |
Data collected for SARE project |
How data will be synthesized |
|
CC Root |
Biomass |
Total C, total N |
Generation of decomposition/ nutrient release curves |
|
CC Aerial Part |
Biomass |
Total C, total N |
Generation of decomposition/ nutrient release curves |
|
Soil |
Soil moisture, microbial respiration, soil food web condition |
Active C |
Correlation analysis of data collected from both projects |
Objective 1 – Classify the C and N release of three annual, fall-planted CC common in the Northeast (oat, field pea, forage radish) and their mixture during spring residue decomposition
Plant tissue samples collected over the course of a two-year (2020-2021; 2021-2022) field experiment conducted at the UMass Crop, Animal, Research and Education Center were processed and sent out for elemental analysis to obtain total C and total N content.
Hypothesis: N stored in both the aerial parts and roots of oat residues, which have a high C:N composition, will be lost quickly at the beginning of the season and over a longer period of time in residues with low C:N ratios such as forage radish and field pea. The aerial parts of field pea and the roots of forage radish will release N more rapidly than their counterparts. Oats will release C slowly throughout the season at a consistent rate, while forage radish and field pea will have a less consistent C release curve. Mixed CC plantings, which have a ‘balanced’ initial composite C:N ratio, will have consistent, gradual C and N release patterns throughout the spring.
Field Experiment: This field experiment was implemented as part of a larger research program with the goal of documenting the biomass decomposition rate of both aerial and root residues of three fall-planted, annual CC species and their mixture from April - July. The experiment was planted in a randomized complete block design and replicated four times. 4 CC treatments were planted in 40 ft2 plots during the first week of September using a plot cone seeder. CC treatments included monocultures of oat, forage radish and field pea as well as a mixture of the three. The mixture was selected based on prior trials conducted in the Hashemi lab.
At the time of winterkill (a killing frost) in 2020 and 2021, 5 linear feet per plot of each CC treatment (both aboveground plant tissue and roots) were harvested and dried in a forced air oven until constant weight. Mesh litterbags (8x8”) containing fresh CC residue were placed in the field at this time and used to track the decomposition of CC roots and aerial parts in the spring. In 2021, bags were collected every two weeks from April – July (12 weeks) and dried in a forced air oven until constant weight. This process will be repeated in 2022.
Laboratory Methods and Statistical Analysis: Dried plant tissue samples (aboveground plant tissue and roots) were ground to a 250 mm particle size and sent to an outside laboratory for elemental (total plant C and total plant N) analysis. The data obtained was used to construct nutrient release curves of both the roots and aerial parts of each cover crop treatment, providing valuable information about the source and timing of soil C and N inputs during spring residue decomposition. Statistical differences were evaluated using the PROC ANOVA procedure in SAS, version 9.4 (SAS Institute, Cary, NC). Discrete, fixed effects included CC species, plant part (root or aerial), and year; sampling date was also a fixed main effect, but a continuous variable. Year is treated as a fixed effect because the experiment is planted in the same location and follows the same plot plan in both years. Regression analysis and mean separation were performed where effects were significant at the p ≤0.05 level.
Objective 2- Quantify changes in soil active C during spring residue decomposition of the three CC species, representing different C:N compositions, compared to a mixture of all three species over the course of two years
Hypothesis: Throughout the season, active C will be most influenced by the decomposition of mixed CC and oat monoculture, which represent a high C:N composition and will provide the greatest overall soil C inputs. Forage radish and field pea monocultures, representing low C:N compositions, will decompose relatively quickly and thus increase active C at beginning of the decomposition period more so than at the end.
Field Experiment: This field experiment was laid out in a randomized complete block design with four replications. Both years, the experiment was planted in the same location according to the same plot plan. CC treatments included monocultures of oat forage radish, and field pea as well as a mixture of the three and a ‘no cover crop’ plot. CC were planted in 40 ft2 plots during the first week of September using a plot cone seeder. Hardneck garlic, an economically important cash crop, was relay-cropped into growing CC in October as part of a larger dissertation project.
Soil samples (20 cores per plot at a depth of 6”) were taken every two weeks during CC residue decomposition in the spring. A portion of each soil sample was analyzed as part of a larger research project (see Objective 3), and the rest was air-dried and archived for future analysis.
Laboratory Methods and Statistical Analysis: Active C, or Permanganate oxidizable C (POXC) (Weil et al., 2003), of the collected soil samples was quantified using the standard operating procedure recommended by the USDA-NRCS (Stott, 2019). Statistical analysis were conducted using the PROC ANOVA procedure in SAS, version 9.4 (SAS Institute, Cary, NC). Fixed, discrete effects include CC species, and year; sampling date, also a fixed effect, was treated as a continuous variable. Year was considered a fixed effect because the experiment was planted in the same location and followed the same plot plan in both years. Regression analysis and mean separation were performed where effects were significant at the p ≤0.05 level.
Objective 3 – Characterize the relationships between cover crop residue decomposition, active soil C, microbial respiration, soil food web status, and soil moisture over the course of two years
Hypotheses: Strong relationships exist between the C and N release of CC residues, active C, microbial respiration, soil moisture and soil food web status. Increased soil active C will be associated with C release from CC residues, increased respiration and an 'enriched’ food web.
Field Experiment: The field experiment described in the previous section (Objective 2) was implemented as part of a larger research program to evaluate the impacts of spring CC residue decomposition (April – July) of the three CC species and their mixture on soil moisture, microbial activity and soil food web structure throughout the decomposition period and compare it with a “no CC” control, which was mulched with straw to prevent weed pressure. At each sampling date, soil moisture was measured using the gravimetric method and the active microbial biomass of each treatment was determined via substrate-induced respiration. The condition of the soil food web was evaluated by nematode faunal analysis, which is used to document changes in soil food web stability, enrichment, and disturbance over time. Soil active C of each sample was quantified as part of this SARE project.
Evaluation Methods and Statistical Analysis: Correlation analysis was conducted using the PROC CORR procedure in SAS, version 9.4 (SAS Institute, Cary, NC). The relationships between soil active carbon, microbial respiration, soil food web status, and soil moisture were documented.
Cover Crop Residue N and C Release Trends
A major goal of this project was to classify the C and N release of three annual, fall-planted CC that differ in composition, C:N ratio and are commonly planted in the Northeast (oat, field pea, forage radish) and their multi-species mixture during spring residue decomposition. Differences in the initial C:N ratios of residue roots and aerial parts can be seen in Figure 1.
The total % N and C lost from residues during the experimental period (November – June) varied by cover crop treatment and year (Figures 2 and 3). Daikon radish (DR) and the multi-species mixture (MX) aerial residues released 79.7% and 68.7% of their total N before June 10, 2021 (Figure 2). This was significantly higher than the proportion of total N released from the aerial residues of oat (OT) (34.7%) in the first year, but not significantly different from the total percentage of N released by field pea (FP) aerial residues (65.0%) which released an intermediate amount of N (Figure 2). In the second year, FP released 82.3% of the total N contained in the aerial residues by June 10, DR released 91.1%, MX aerial residues released 82.2%, and OT aerial residues released 64.5% of their total N by the end of the experimental period (Figure 2). The proportion of N released by DR aerial residues was significantly higher than the proportion released by OT; the proportion of N released from the other treatments did not differ significantly from each other. FP and DR released 81.8% and 79.7% of their total root N by June 10 in the first experimental year, significantly higher than the amount released by OT and MX root residues, 62.6% and 69.1%, respectively (Figure 2). OT roots released a significantly smaller proportion of total root N (26.7%) than all other cover crop root residues in the second year (>80%) (Figure 2).
Figure 2. Percent N remaining in cover crop aerial and root residues during spring decomposition.
The percentage of total aerial C released by DR in the first year (72.1%) was significantly higher than the percentage released by aerial OT residues (45.0%) (Figure 3). FP and MX released intermediate amounts of the total C found in their aerial residues (57.9% and 64.5%, respectively) in the first year, proportions that did not differ significantly from the other treatments (Figure 3). In the second year, DR released a significantly greater proportion of total aerial residue C by the end of the experimental period (92.7%) than the aerial residues of all other treatments (<56%), which did not differ significantly from each other (Figure 3). The percentage of total root C released by the residues by the end of the experimental period was significantly different in the second year, but not in the first. In the second experimental year, DR roots released significantly greater proportions of total residue C (93.3%) than all other treatments by June 10, while OT released a significantly smaller proportion of total root C (48.8%) than the other treatments (Figure 3). FP and MX root residues released intermediate amounts of their total root C before June 10 of the second experimental year (71.5% and 79.8%, respectively), values that were significantly different from DR and OT but not from each other (Figure 3). Overall, DR had the highest rate of N and C release, OT had the lowest, and FP and MX release rates were intermediate.
Figure 3. Percent C remaining in cover crop aerial and root residues during spring decomposition.
Estimates of total N (kg ha-1) and C (kg ha-1) released from cover crop residues were calculated using the N and C yield (kg ha-1) of residues adjusted by the percentage of cover crop residues remaining at the end of the spring sampling period, averaged between the two experimental years. OT residues released significantly less N (18 kg ha-1) during the experimental period than the other cover crop residues, whose N release ranged from 48 kg ha-1 to 50 kg ha-1 . DR and MX released significantly more C than OT and FP during the experimental period, although the majority of DR C was released from root residues, while the majority of MX C was released from the aerial residues. Overall, our results suggest that while planting cover crops can provide a substantial amount of N to subsequently planted spring crops, species selection and time of cash crop planting are important factors in determining the amount of N available to spring planted crops. Moreover, our results indicate that a portion of residue N is released before April 1 and is likely lost to the environment, thus making it unavailable for subsequent crop uptake. The amount of N released by residues in the winter is likely influenced by weather conditions, including temperature and precipitation, during the winter period. As such, N from spring cover crop decomposition should not be treated as a consistently reliable source of supplemental N for spring cash crops. In general, the majority of N present in the aerial residues of FP and DR was released during the winter period, while the majority of N found in their root residues was released during the spring decomposition period, between April 1 - June 10. By contrast, the N released from MX aerial and root residues followed similar trends both years. OT residues, which decomposed more slowly and had the most residue left of any of the treatments by the end of the experimental period, continue to release C and N throughout the summer. However, due to their high C:N, OT shoots may cause temporary net N immobilization in the soil. The influence of oat cover crop residue amendments on soil N should be studied along a longer time period to better understand their contributions to soil fertility.
Changes in Cover Crop Residue C:N During Spring Decomposition
The C:N of cover crop shoot residues did not fluctuate significantly over the decomposition period . However, sampling date significantly influenced the C:N of root residues during spring decomposition. OT root C:N decreased linearly both years (Figure 4). The sampling date by year interaction was significant for FP, DR, and MX root residue C:N (Figure 4). Despite significant changes observed during spring decomposition, the root C:N of FP and DR remained lower than the accepted residue C:N immobilization threshold (24:1), driving net N mineralization in the soil. The greatest change was observed in OT root residues, the C:N of which decreased steadily throughout the spring but remained high, suggesting that oat roots contribute to N immobilization dynamics in soils during decomposition, despite changes in chemical composition. Changes in mixed residue root C:N during spring decomposition were variable, dropping below the 24:1 C:N threshold within the first four weeks of decomposition in the first year but remaining higher in the second year. These results emphasize the variability of cover crop mixture decomposition, indicating that further research is necessary to better understand the complexities of this variation and how it relates to management and climate fluctuations.
Figure 4. Changes in C:N of cover crop root residues during spring decomposition.
Influence of Cover Crop Residues on Soil Active C During Spring Decomposition
The year by sampling date interaction significantly influenced permanganate-oxidizable C (POXC) levels in this study. POXC fluctuated significantly during the sampling period of the first experimental year (Figure 5). From the first sampling date (April 1) until early May, POXC levels decreased, followed by a sharp increase in POXC that remained relatively steady for the remainder of the experimental period. In the second experimental year, POXC levels did not differ significantly throughout the sampling period and were overall significantly higher than in the first experimental year (338.95 and 398.40 mg kg-1, respectively) (Figure 5). POXC levels were not significantly impacted by the species of fall cover crop planted in either year. It is worth noting that a summer buckwheat cover crop was planted in the experimental field before the beginning of the second experimental season, possibly contributing to the overall increase in POXC in the second year. Overall, our results indicate that either 1) POXC is not a sensitive enough indicator to capture short-term differences in soil health in response to annual cover crops, or 2) two years is too short of a time period to see marked improvements in soil health in response to annual cover crop amendments.
Relationships Between Soil Active C, Microbial Respiration, and Nematode-Based Indices (NBIs) of Soil Food Web Condition
The third objective of this project was to characterize the relationships between two commonly used soil health indicators, soil active C and microbial respiration, and NBIs of soil food web condition. Relationships between selected indices, soil respiration and active C are presented in Figure 6.
Overall, the relationships between active C and the NBIs were very weak, indicating that active C can not be used as a proxy indicator of soil food web condition. Although weak, the correlations between soil respiration and the selected NBIs were generally stronger than those between active C and the NBIs, suggesting that microbial respiration is more closely related to soil food web indicators than active C, although it is still unlikely that respiration can provide relevant information about soil food web condition on its own. The relationship between Channel Index (CI), which represents the ratio of fungal:bacterial feeding nematodes, and soil respiration was moderate, suggesting that soil respiration has the potential to provide some knowledge about the primary decomposition pathway of a soil food web. Future research should focus on elucidating the relationships between microbial community composition and respiration in order to broaden the usefulness of microbial respiration as a soil health indicator. Overall, our results indicate that while the use of NBIs can provide relevant information about soil food web function and condition, these analyses cannot yet be readily integrated into the existing soil health framework and must be evaluated independently.
- The roots and shoots of annual, fall planted cover crops release N at different rates
- Daikon radish residues undergo almost complete decomposition by early June, releasing the majority of N and C contained in the residues. While this may effectively recycle nutrients in early spring, it likely does not provide good synchrony for N uptake of early summer planted crops.
- The N release from oat residues is more gradual than that of field pea, cover crop mixture, or daikon radish residues and continues past the spring
- N release trends from fall planted, annual cover crops vary from year to year; thus, farmers should use caution when anticipating N contributions from cover crops to offset N fertilization in the spring and early summer. On average, field pea, forage radish, and mixed cover crop residues released close to 50 kg ha-1 N in the time before June 10, while oat residues released only about 18 kg ha-1 N.
- Annual, fall planted cover crops provide substantial amounts of C to soil systems, and the C release rate from residues is generally less variable from year to year.
- Active C, a commonly measured indicator of soil health that is especially responsive to short-term management changes, is not significantly impacted by cover crop species during spring decomposition after two consecutive years of planting.
- Nematode-based indices (NBIs) of soil food web condition can provide useful information about soil function; however, these analyses are not readily correlated with microbial respiration or active C, two commonly measured soil health indicators. Future work should focus on identifying ways to integrate soil health indicators and NBIs to provide more comprehensive interpretations of soil health and how it is impacted by best management practices.
Education & Outreach Activities and Participation Summary
Participation Summary:
The results of this project were disseminated via workshops and annual meetings that included farmers, agricultural professionals, and home gardeners; these workshops included the ASA-CSA-SSSA Annual Meeting (November 2022), NOFA MA Winter Meeting (January 2023) and the Certified Crop Adviser Annual Meeting (January 2023). The major outreach goals of this project were to 1) to provide growers with specific information about how fall-planted CC impact soil health and nutrient cycling in cropping systems and 2) to promote a broader understanding of the interactions between soil biology, soil health, and CC management on farms.
Project Outcomes
This project will contribute to agricultural sustainability by providing new information about the N and C contributions of commonly planted cover crops in the Northeast. Specifically, our results indicate that fall planted, annual cover crops can release a substantial amount of C into soil systems, necessary for soil organic matter formation, but are unlikely to be reliable sources of supplemental N for spring planted cash crops. Moreover, this project demonstrated that two consecutive years is not enough time to see short-term soil health differences (as measured by active C) in the Northeast; as such, farmers who are interested in soil health testing may prefer to stagger these evaluations over a longer period of time, rather than submitting samples for soil health evaluations annually.
In addition to enhancing our laboratory skillset by allowing us to develop a new soil health protocol for use in our lab, this project influenced our knowledge and attitude about sustainable agriculture in the following key ways:
- Sustainability and soil health can be defined in many ways, and often the metrics we use to evaluate soil health improvements or soil function are distinct and not easily integrated. Learning to think of soil systems and sustainability in a dynamic way can provide more comprehensive assessments in the long run, despite the challenges that come along with integrating different types of data.
- Sustainability and long-term resiliency should be considered separately from short-term benefits on farms. For example, this study showed that the spring nitrogen (N) contributions of annual, fall planted cover crops are variable and likely influenced by annual weather conditions, rendering them insufficient to reliably offset N fertilization to successor crops. However, cover crop residues consistently add substantial amounts of carbon (C) to the soil, which contributes to soil organic matter formation, and thus improved soil N retention, in the long-term. As such, it is important to mitigate short-term expectations with long-term sustainability goals when thinking about overall soil health and resiliency.