Soils play important functions in sustaining crop productivity, maintaining plant, animal and human health, and providing various ecosystem services. However, intensive management practices, land use conversion, and adverse climatic conditions can negatively impact soil properties and crop production. Therefore, there is a strong need to protect and improve soil quality, which requires identifying the costs and benefits of existing agricultural practices, understanding the optimal conditions for implementation of more sustainable soil management practices, selecting the best management practices available, and developing and testing new tools. The overall aim of this project is to clarify location-specific and general challenges to maintain soil health under intensive management, evaluate how soil management and land use decisions impact soil health under current and future climate conditions, and inform on practices that enhance soil health through maintaining or increasing  soil organic carbon (SOC) stocks, reduction of greenhouse gas emissions, improved productivity and other ecosystem services. Of particular interest are agricultural practices that increase or maintain SOC stocks since SOC is tightly associated with multiple soil health parameters.

Soils are the largest terrestrial carbon pools (Friedlingstein et al., 2019), making soil carbon sequestration a potential, though debated, tool in reducing atmospheric CO2 (Minasmy et al., 2017; Amelung et al., 2020). SOC also impacts multiple soil physical, chemical, and biological parameters and is therefore a cornerstone to various soil functions and ultimately crop productivity. Having been identified as a key soil health indicator, SOC has become a shorthand for soil health (Bagnall et al., 2023). Yet, intensive agricultural practices often (i) accelerate decomposition and loss of existing SOC stocks, (ii) limit the rate of C input, and (iii) reduce the persistence of C inputs. Thus, increasing SOC stocks to improve soil health requires adoption of sustainable soil management practices. Although the mechanisms for SOC change are increasingly understood, our ability to predict it in different ecosystems and under different climate forcing conditions remains limited.   

One of the important functions of SOC is promoting aggregation, which facilitates soil water and nutrient holding capacity. In return, soil aggregation can increase SOC storage by physically protecting carbon from mineralization by microbes through encapsulation in smaller pores and by reducing soil erosion (Razafimbelo et al., 2008; Six et al., 2002). SOC dynamics and aggregate (including stability and size distribution) interactions are thus clearly important for microbially driven biogeochemical processes (e.g., greenhouse gas emissions) and climate change mitigation (Blaud et al., 2012; Rillig et al., 2016; Vos et al., 2013; Wang et al., 2019). Soil organic matter (of which ~50% is C) is also critical for soil fertility and plant nutrient availability as it provides an energy source for microbes and serves as a store for nutrients (e.g., N and S) used directly by plants or indirectly following mineralization and cycling by soil microbes. Studies have shown direct relationships between SOC and yield. For example, Oldfield et al. (2022) found positive correlations between soil organic matter and crop yields across 49 farms in Wisconsin and Minnesota. The authors suggested that SOC-driven nutrient availability was partially driving this relationship.

Agricultural management practices have important impacts on soil health through their effects on SOC storage and aggregation. However, long-term pedogenesis processes and short-term economic production objectives can at times conflict with each other. The management decisions in intensive agriculture production largely consider short-term goals based on machinery operation, seed bed preparation, and application of agrochemicals to maximize immediate grain or forage yields. These factors in conjunction with varying seasonal temperatures, water availability, and market demand determine which crop to plant and tillage system to use, and the farm’s potential productivity. However, in the long-term, intensively managed soils negatively impact SOC accrual and soil health (Dalal and Chan, 2001). At the same time, evidence is accumulating that different microbial guilds in the soil also facilitate SOC accumulation, and intensive management techniques appear to undermine the C-accrual process. For example, fertilization reduces plant dependence on mycorrhizal partner fungi, which in turn reduces competition in the soil between symbiotic and saprotrophic fungi, leaving more mineral resources for the latter and ultimately facilitating faster decomposition of SOM (Keiluweit et al. 2015; Pellitier et al. 2021; Carrara et al. 2021, 2022; Noormets et al. 2023). Accelerated decomposition and declining SOC undermine, in turn, the many functions provided by soil aggregation.

A number of climate-smart agriculture practices (e.g., no-tillage, cover cropping, residue retention) facilitate positive soil carbon balance by not only increasing detritus (and thus C) input to the soil, but by maintaining continuous live root crop, also maintain a continuous symbiotic microbial community, which contributes to stabilizing SOC stocks through competitive inhibition of the saprotrophs (Brzostek et al. 2015, Keiluweit et al. 2015, Peršoh et al. 2018). This interaction between plants and different microbial communities appears to be at the heart of the reduced greenhouse gas emissions and SOC accumulation associated with  improved soil health (Paustian et al., 2016). The effectiveness of these climate-smart agriculture practices depends on environmental factors and management conditions. However, the high degree of uncertainty in the outcomes is a key limitation to adopting these management practices. This multi-state project  provides an excellent opportunity to reduce this uncertainty by advancing our understanding of the agronomic and environmental benefits of climate-smart agriculture practices in diverse cropping systems across the United States, thus informing soil management decisions that enhance soil health and agroecosystem resilience across agroecological zones. 

Enhancement of SOC stocks can be achieved through (i) decreasing SOC decomposition resulting from tillage and erosion (Cai et al., 2022), (ii) increasing C inputs through increased photosynthesis or application of exogenous C inputs (Berhane et al., 2020), and (iii) increasing the persistence of existing and new C, through leveraging (1) soil microbial competition as outlined above and (2) the  application of C stabilizing amendments such as biochar, both of which modify the relative availability of C and N, and thus suppress  SOC priming (Connan et al., 2020; Weng et al., 2022). This project will focus on evaluating the breadth of effects that current soil conservation practices have on SOC dynamics, with the objective of improving the predictive capability of the effects in different production systems and under different climatic conditions.  Such practices in cropping systems include, but are not limited to, cover cropping, reduced or no tillage, residue retention, application of C-enriched amendments such as manure, compost, and biochar, and integration of livestock. In forest systems, the SOC response will be described under alternative site preparation, prescribed burning and competition control treatments. On grasslands, improved grazing management practices such as multi-paddock grazing (Kim et al., 2023; Teague & Kreuter, 2020) and regenerative practices such as pasture cropping, which integrates direct seeding of annual crops into dormant perennial grasses (Miller & Badgery, 2009), are found to increase SOC stocks and improve soil health. 

Cover crops have become increasingly more widespread as a tool to mitigate soil erosion, retain or increase (with legumes) nutrients and increase carbon inputs (Robačer et al., 2016). Cover cropping has been shown to improve soil aggregate stability, increase SOC content, and improve nutrient utilization efficiency (Blanco-Canqui et al., 2015; Brennan and Acosta-Martinez, 2017), all of which result in less resource inputs with desirable environmental and economic outcomes. In addition, cover crops, either as living plants or dead plant residues, have direct and indirect impacts on soil microbial communities (Brennan and Acosta-Martinez, 2017; Finney et al., 2017). However, the exact mechanism of the effect of cover cropping on SOC dynamics remains incompletely understood. The current project aims to elucidate SOC response to root dynamics, plant-microbe interactions, , biomass production potentials, the quantity and quality of root exudates, the C:N ratio of the biomass and soil medium (Finney et al., 2017; Tiemann et al., 2015), and the taxonomic and functional diversity of the soil biota (e.g., Olson et al., 2014, Poeplau and Don, 2015). . 

Diversification in terms of row crop rotation can also result in positive agronomic and ecological outcomes in production systems. Crop rotations affect the quantity and quality of crop residue returned to the soil and thus can influence microbial activities and functions pertinent to nutrient and C cycling (Ashworth et al., 2014; Bremer et al., 2011; Sainju et al., 2014). Based on published literature, annual corn–soybean rotations increase yield compared to their respective monoculture sequences (Mannering and Griffith, 2007; Wilhelm and Wortmann, 2004). However, a three-year rotation of corn-soybean-wheat did not result in yield advantage (Lund et al., 1993). Crop rotation can also reduce the occurrence of weeds, insects, and diseases (e.g., Higgs et al., 1990; Chen et al., 2001; Howard et al., 1998) and alter SOC accumulation (Halvorson and Schlegel., 2012). A study conducted in Tennessee showed that surface C storage can be enhanced by crop sequence diversity in no-till systems after 12 years, whereas subsurface C levels may require more time to be influenced (Ashworth et al., 2014). 

A major risk in crop production in the United States is drought. The United States is experiencing increasing seasonal and annual variability in rainfall, which can lead to more frequent soil water deficits during the growing seasons, such as the droughts in 2023. The importance of soil water retention to agronomic productivity cannot be over-emphasized and soil organic matter is a critical determinant of water retention in the root zone (Stewart and Lal, 2018). Increasing soil organic matter can improve water infiltration and enhance water holding capacity by improving soil structure (Hudson, 1994; Franzluebbers, 2002; Lado et al., 2004; Minasny and McBratney, 2018). Soils with higher organic matter can retain more water under vapor pressure deficit, protecting crops from losses induced by drought better than low organic matter soils (Iizumi and Wagai, 2019; Carminati and Javaux, 2020). An increase in soil organic matter by 1% would increase plant available water capacity by 1-3% (Libohova et al., 2018). Every 1% increase in organic matter results in as much as 25,000 gallons of soil water storage per acre.  Increasing soil organic matter content may increase tolerance to short-duration drought stress during the growing season. Adhikari et al. (2021) suggested that corn yield during a drought year in Texas could have been explained by differences in soil health.  Many areas of the NC are affected by drought, and more frequent storm events. Therefore, building up soil organic matter through improved management practices to maintain soil moisture at levels that enable crops to withstand water deficit is a crucial component of increasing the resilience of agricultural systems and protecting crop yields (Lal, 2020).

An important test of the effectiveness of the climate-smart practices is their interaction with climate change factors like temperature and water availability. Higher mean temperatures and increasing frequency of droughts cause significant global disruptions in agricultural production (Wiebe et al. 2015). Often, though, high temperatures coincide with water limitation and the net effect on crop yield and SOC pools may be difficult to disentangle. However, there are signs that simplistic assumptions about the similarity of productivity and SOC responses to these interacting climate factors may not be justified. For example, Noormets et al. (2021) showed that the simultaneous suppression of carbon assimilation and respiration lead to increased net carbon balance (carbon sequestration) under drought. Furthermore, changes in carbon allocation to different tissues and their chemical composition under changing resource availability and stress levels may expose additional surprises and feedbacks in SOC dynamics (Adamczyk et al. 2019, Augusto and Boča 2022, Carrara et al. 2021, Finzi et al. 2015, Schmidt et al. 2011b).  

Rationale 

This proposal outlines a project designed to help better understand changes in soil health under intensive management and optimize decision making related to alternative climate-smart practices to enhance soil health and SOC stocks. The project participants have previously shown how soils can be conserved and their quality maintained or improved by timely decisions and adequate soil management interventions. We seek to build on and expand the work of the predecessors of this project over the past four decades of research to design, promote and implement best practices to build more resilient soil and agroecosystems under future climate change scenarios. We view this approach as a natural progression related to the past research of NC-174, NC-1017, and NC-1178. The focus will be on critically evaluating the evidence of land management techniques on different stages of SOC dynamics, and potential modifications to increase SOC stocks and improving soil structure to enhance soil function, reduce greenhouse gas emissions, and simultaneously devise locally relevant mitigation and adaptation strategies to future climatic scenarios. This multi-state project spans researchers from across several agroecological and climatic zones which positions us to generate insights and provide guidance relevant to multiple scenarios, which is a stepping stone for widespread adoption of climate smart practices. Knowledge gained from the proposed research will contribute to a more quantitative understanding of the effects of intensified vs. sustainable agroecosystem management on SOC stocks, soil structural changes, and soil health and ecosystem services over the varying climates and soil landscapes that occur in the participating states.