Small soil C cycle responses to three years of cover crops in maize cropping systems

Highlights

• Cover crops did not alter maize residue decomposition or microbial community composition.

• Rye and bluegrass cover crops increased PMC and bluegrass increased POM.

• POM-C and –N were correlated with total residue and PMC was correlated with NPP.

Abstract

Cover crops are touted for their ability to improve many ecosystem functions in annual cropping systems. In addition to water and nutrient retention, cover crops may influence C cycling by increasing total C inputs to the agroecosystem, stimulating microbial populations, altering main crop residue decomposition rate, or changing litter chemistry over time. We assessed whether annual (rye) or perennial (bluegrass) cover crops in maize cropping systems influenced maize residue decomposition (litterbags) or microbial communities (shotgun metagenomics) in soil and litter, and whether these cover crops had an effect on microbially active pools of C: particulate organic matter (POM) C and N, or potentially mineralizable C (PMC) after three years of cover crops. Neither cover crop affected litterbag decay rates or microbial composition relative to no cover crop controls. However, both cover crop types increased PMC indicating that microbially-available C was boosted by cover crops. Total POM and POM-N were higher with bluegrass cover crops. These modest effects of cover crops on dynamic soil C pools suggest the generally positive long-term effects of cover crops on soil protection and nutrient retention are related to incremental shifts in quantity, timing and quality of C inputs.

Introduction

Cover crops have been reported to increase mean soil organic C (SOC, Poeplau and Don, 2015) but not to increase overall net ecosystem exchange (Baker and Griffis, 2005; Gebremedhin et al., 2012). This discrepancy likely stems from the myriad and conflicting effects that cover crops may have on components of the C cycle and subsequent SOC stabilization in annual agroecosystems. Cover crops could have a positive effect on SOC by a) increasing total C inputs through addition of litter (Austin et al., 2017), b) increasing microbial pools of C (Kallenbach et al., 2015), and/or c) theoretically, decreasing decomposition rate of main crop residue, although this has not been observed. Alternatively, they can have a negative effect on SOC by a) priming decomposition of native SOC (Zhu et al., 2014) and/or b) increasing the decomposition rate of main crop residue (Varela et al., 2014). Depending on the season, topography, and crop rotation, one of these processes may dominate the response of SOC to cover crops, leading to the wide variation seen in SOC response to cover crops (Muñoz et al., 2014; Poeplau and Don, 2015).

Cover crops represent a pulse of C into the system when they are terminated and left to decompose in place, the most likely way they may increase SOC. The decomposition of this litter will result in CO₂ release, leaching of soluble C compounds through the soil profile and transformation of litter fragments into POM (Cotrufo et al., 2015). Depending on the litter and soil type and disturbance level, a small quantity of litter C may be physically incorporated in soil aggregates and sorbed on mineral surfaces, leading to physical protection and SOC turnover times on the order of decades, centuries, and millenia (though SOC turnover times vary among soils and climates, Castellano et al., 2015; Cotrufo et al., 2015; Lehmann and Kleber, 2015).

Importantly, cover crop biomass C may be much less than main crop residue C inputs, rendering the potential SOC increase by this pathway negligible (Cates and Jackson, 2018). However, by shifting temperature and moisture patterns on the surface, living or dead cover crops may affect the decomposition of main crop residues, (Chen et al., 2018; Dijkstra et al., 2010, 2009; Flerchinger et al., 2003; Varela et al., 2014). In addition, there is the potential for cover crops to alter mesofauna composition, altering the decomposer abundance, community, and pathway for residue decomposition (Blubaugh et al., 2016; Leslie et al., 2017) although this is not always observed (Fox et al., 2016). Residue and soil microbial composition also changes with cover crops (Barel et al., 2019; Mbuthia et al., 2015), which may signal shifts in residue decomposition processes (Bray et al., 2012; Schneider et al., 2012). Maize residue decomposition rate is of particular interest in northern climates as soil warms more slowly under large amounts of residue (Andraski and Bundy, 2008; Vanhie et al., 2015) and soil N is immobilized as residue decomposes (Green and Blackmer, 1995). Growers who are concerned about spring soil temperatures harvest, incorporate, or fertilize maize residue in an attempt to speed decomposition, despite limited efficacy (Al-Kaisi et al., 2017; Andraski and Bundy, 2008) or burn residue in place (McCarty et al., 2009). This is in direct opposition to recommendations for retaining residue to build SOC (Johnson et al., 2014) and unlikely to move agricultural lands toward the suggested 4 per mille annual increase in SOC to offset greenhouse gases (Minasny et al., 2017).

Finally, cover crops may alter SOC by stimulating the microbial community with simple C compounds from their roots. These root exudates can “prime” decomposition of native SOC by providing easily-decomposed C to the rhizosphere microbial community, which may subsequently increase or decrease decomposition of native SOC (Dijkstra et al., 2009; Zhu et al., 2014). According to metanalysis, rhizosphere priming reduced SOC by 59% on average, especially early in plant growth, but this effect was limited in crops (Huo et al., 2017). Beyond shifting rates SOC decomposition, rhizosphere priming may increase microbial biomass and shift the composition, and perhaps the function, of microbial communities (Cheng, 2009). For example, rye cover crop root exudates were found to constitute a substantial input of bioavailable C and stimulated growth of microbial biomass C (Austin et al., 2017), which may be the basis for some stable SOC (Kallenbach and Grandy, 2015). There is a need to clarify whether stimulating microbial activity with cover crop root exudates is likely to increase SOC from increased C inputs to the soil environment or decrease SOC as a result of rhizosphere priming. One method to investigate these dynamics is to assess dynamic C pools such as potentially mineralizable C (PMC) and particulate organic matter (POM), which are considered early indicators of SOC change stemming from agronomic management. Both PMC and POM have been shown to increase under cover crops, perhaps because of increasing microbial activity (Ladoni et al., 2016; McDaniel et al., 2014; Snapp and Surapur, 2018).

In a previous study, we evaluated the effects of rye (annual grass) and bluegrass (perennial grass) cover crops on net ecosystem C balance (NECB) in continuous maize and found no difference in NECB with cover crops compared to no cover, indicating cover crops did not increase C inputs relative to C losses during the three year study (Cates and Jackson, 2018). In the same study, bluegrass and rye both increased early-season rates of heterotrophic respiration, suggesting that either rhizosphere priming or cover crop root exudate mineralization and raising questions about how cover crops may stimulate microbial mineralization of soil C. Here, our objective was to evaluate whether increased SOC accumulation under cover crops, via slower decomposition of maize residue and accumulation of microbially-available C, may be possible despite limited C inputs from cover crop litter. We measured decomposition of maize residue, metrics of labile C (PMC and POM), and SOC stocks after three years of rye or bluegrass cover crops in a continuous maize cropping system. To characterize decomposition with and without cover crops, we evaluated litter chemistry and microbial composition in surface litter and adjacent soil. Building on the previous work, we also explored correlations between C inputs and active C metrics to illuminate what the C sources of these active C pools might be.