Contributions of legume cover crop root systems to soil carbon pools in organic systems using different termination strategies

View the project final report

Commodities

• Agronomic: clovers

Practices

• Crop Production: cover crops

In the field study, legume root decomposition and N release were not affected by termination approach or soil inorganic N levels.  All roots decomposed similarly at the Goldsboro, NC site, but crimson clover roots decomposed fastest at the Kinston, NC site.  At both sites, hairy vetch roots released N fastest and crimson clover roots released N slowest.  In the incubation, legume fine roots decomposed and released N faster than coarse roots, but species and N addition did not affect N release.  Our results indicate that under the humid summer conditions of the Southeast, legume roots decompose and release N rapidly.

Introduction

The purpose of this project was to investigate the decomposition and N release dynamics  of legume cover crop roots, and how this may be affected by cover crop management and root morphology.  In particular, we investigated how popular and novel termination methods interact with three winter annual leguminous cover crop species’ root morphologies to ultimately result in root decomposition and N release.  Soil contains more carbon (C) than plants, animals and the atmosphere combined. This Soil organic carbon (SOC) is a critical part of agricultural soils due to its ability to improve soil physical properties such as water holding capacity and aggregate stability, leading to deeper and more prolific root systems (Alcántara, 2011) and creating a positive feedback loop where larger diameter roots contribute further to increasing SOC. With the need to adapt to a changing climate, and potential payment for C storage looming on the horizon, farmers are particularly interested in learning more about management strategies that will help contribute to SOC sequestration. Organic systems can have a particularly important role in C storage due to the many C-based inputs utilized.  Winter annual legume cover crops, such as Austrian winter pea, crimson clover, and hairy vetch, are typically planted in late fall and terminated in spring before summer crops are planted.    Cover crop roots generally remain in the soil profile following termination, making them an important potential contributor to long-term SOC.

While most cover crop research has focused on shoot material, roots deserve further attention considering that C contributions from root biomass can be significantly large.  Shoots generally have lower C to nitrogen (C:N) ratios and quantities of recalcitrant materials than roots, leading to faster decomposition than root material (Bird et al. 2008, Wang et al. 2010), and suggesting that root-derived C may contribute greatly to overall soil C stored after a season of cover crop growth.  Puget and Drinkwater (2001) reported hairy vetch shoots to have a higher N content (4.5 vs. 2.8%), lower C:N ratio (9.7 vs. 11.9), and lower lignin concentration (5.2 vs. 17%) than roots, favoring more rapid shoot decomposition.  Buchanan and King (1993) report similar findings with crimson clover.  Combined, available data indicate that legume cover crop shoots may decompose more rapidly than roots (Fujii and Takeda, 2010). This observation has been verified in field studies demonstrating that 50-52% of root derived cover crop C can remain in the soil, compared to only 4-13% of the quickly-decomposed shoot C, at the end of the growing season  (Puget and Drinkwater, 2001; Kong and Six, 2010). Over the long term, partially-decomposed root material has been shown to be preferentially stored within aggregates, giving this material increased physical protection as root derived particulate organic matter (Alcantara, 2011). Further, the continuous nature of root carbon inputs from exudates and fine root turnover leads to a constant input of root carbon into the soil environment (Puget and Drinkwater 2001).

Leguminous cover crops are an important feature of organic systems and can provide significant quantities of plant residues that build up SOC and furnish essential plant nutrients, especially N, for subsequent crops.  In fact, the national organic standard singles out the role of cover crops in these systems, stating that growers should “Manage crop nutrients and soil fertility through rotations, cover crops, and the application of plant and animal materials” (NOP, 205.203 (b).  Investigating the decomposition and and N release of legume cover crop roots under different  management scenarios will provide producers with pertinent information to better utilize leguminous cover crops.

Decomposition rate is controlled by many factors, including soil N availability, and size of the decomposing material.  In N-limited (low soil N) systems, the amount of N present for microbes to use as a nutrient source during the decomposition process controls their rate of decomposition, with N shown to be a limiting factor in fine root decomposition for tree species (Lin et al. 2010).  Since most agricultural systems are N-limited, any increase of N to the soil via N-rich materials, such as incorporated legume shoots, will stimulate decomposition of available carbon substrates, including roots. In this project, we  conducted a field study utilizing two cover crop termination methods that are either commonly employed by organic growers (incorporating residue), or that are novel, with organic producers desiring further study (roller-crimping). These methods place N-rich shoot biomass at different locations in the soil profile. For example, incorporation may enhance decomposition due to increases in N from the shoots.  Roller-crimping leguminous cover crops, however, may slow decomposition since N is concentrated in a mulch on the surface rather than being in contact with soil decomposer microorganisms. We hypothesized that incorporated residues will increase root decomposition and N release rate compared to roller-crimped legumes due to an increase in N available to the decomposer microbes.

Root morphology, particularly diameter, has been shown to influence decomposition and nutrient release (Fujimaki et al. 2008). Since nutrient release from legume roots of different size classes is not reported in the literature, morphological characterization would be valuable . Further, the effect of soil N status on decomposition of different root sizes has also not been investigated. Thus, a second major objective of this project was to assess root morphology and conduct a laboratory-based decomposition study over an N gradient to provide information pertinent to producers seeking to predict how N fertility in their fields will affect long-term root C persistence.   We hypothesized that smaller roots would decompose and release N faster than larger roots with any increase in N inputs.

This research has generated data for evaluating the residence time of legume cover crop root biomass and root N release dynamics, which can give producers a better idea as to which legume cover crop roots are most effective at persisting in soil and releasing N.  Furthermore, this project’s emphasis on investigating the long-term residence time of legume cover crop root biomass and root N release dynamics in agricultural fields is in accordance with the SARE program objective of promoting stewardship of natural resources and maintaining the profitability of sustainable agriculture.

1. Determine the effect of winter annual leguminous cover crop termination method (incorporation vs. roller-crimping) on soil carbon by altering available soil N at rooting depth.
2. Determine how cover crop root size (diameter) affects decomposition across a soil N gradient created in a lab incubation study.