Abandoned agricultural fields have received growing research interest, because an increasing number of degraded agricultural lands are abandoned, especially in developed countries (Lark et al., 2020; Rey Benayas, 2007) and these lands have great potential for soil C sequestration (Knops & Bradley, 2009; Knops & Tilman, 2000; Post & Kwon, 2000). When lands are used for agricultural production, soil disturbance accelerates SOM decomposition (Conant et al., 2001; Knops & Tilman, 2000), leading to greenhouse gas emissions (Hutchinson et al., 2007). Once agricultural lands are abandoned, and soil disturbance ceases, the soil C stocks can recover to the previous level(Jones & Donnelly, 2004; Post & Kwon, 2000). However, widely varying rates of C accumulation have been reported (McLauchlan et al., 2006), and the mechanisms controlling soil C accumulation in abandoned agricultural fields remains poorly understood.
To date, much of the research on C sequestration in abandoned agricultural fields has focused on the surface soil (Jia et al., 2020), and much less is known about deeper depths (Rumpel & Kögel-Knabner, 2011). The dynamics of subsoil C may even be more important than surface soil C (Rumpel & Kögel-Knabner, 2011). About 50% of the C stocks in the first 1 m of soil in temperate grasslands are stored between 30 and 100 cm (Jobbágy & Jackson, 2000). Any change of this amount of soil C can have significant impact on the fluxes of atmospheric CO2 in the abandoned agricultural fields. Yet, our knowledge of the direction of change in subsurface SOM, as well as the factors influencing subsoil C, are limited.
Our understanding of the factors that control C dynamics in surface soil might not apply to the subsurface soil, as C inputs and SOM stability are different between the surface and subsurface soil (Wordell-Dietrich et al., 2017). For subsurface soil, important sources of C inputs include deep roots, dissolved organic carbon (DOC) and bioturbation (Rumpel & Kögel-Knabner, 2011). Carbon flux from roots to the soil is much lower in the subsoil as compared to the surface soil, as the root biomass decreases sharply with depth, especially in grassland ecosystems (Jackson et al., 1996). The quality and quantity of DOC in surface soil and subsoil are also found to be different (Kaiser & Guggenberger, 2000). The importance of DOC as a C input into subsoil depends on precipitation, which controls the flux of DOC, and soil retention, which controls the quantity of DOC that remains in the subsoil (Ahrens et al., 2015). Recent research also suggests subsurface SOM stabilization is influenced by the availability of labile substrates, i.e., the priming effect (Fontaine et al., 2007). However, the direction of the priming effect, as well as its controlling factors, are still poorly understood (Kuzyakov, 2010). In addition, changes in temperature and nutrient availability also impact subsoil C dynamics (Fierer et al., 2003). This variability highlights the need for improving our understanding of the dynamics of the C stocks in the soil below 20 cm to obtain accurate estimates of the total soil C storage potential of abandoned agricultural areas.
To understand the dynamics of both surface and subsurface soil C after agricultural abandonment, we conducted repeated soil survey in a chronosequence of 21 old fields over 13 years at Cedar Creek Ecosystem Science Reserve (CCESR), Minnesota, which has a long history of monitoring soil C and N dynamics after agricultural abandonment from crop cultivation. Knops and Tilman (2000) using a repeated survey of surface soil of the same chronosequence, demonstrated that soil C stocks decreased by 89% and soil N decreased by 75% during the agricultural period, and predicted that it required 180 years for C and 230 years for N to recover to pre-agricultural levels. They also concluded that plant functional group composition significantly affects the accumulation rate of both soil C and N. However, the study of Knops and Tilman (2000) was based on only the top 10 cm of the soil depth. Knops and Bradley (2009) subsequently conducted a one-time inventory of soil C and N in the same old fields down to 1 m with six depth intervals, and they found C and N accumulated in the surface soil yet based on a chronosequence, which did not show significant trends in the subsoil. This could be attributed to spatial variability among fields in soil condition at the time when the old fields were abandoned and slow C and N change rates in the subsoil. One long-term repeated soil survey in an old field with prescribed fire treatment at CCESR (Li et al., 2014) showed soil N stock in the 20–100 cm depth interval decreased over the 10-year sampling period. However, this result might not be representative for all CCESR old fields, since the study was not replicated at the field level, and fields with a similar abandonment age vary greatly in total soil C (Knops & Bradley, 2009). Finally, a recently published study (Yang et al., 2019) of a fenced and species richness controlled experiment at CCESR demonstrated that soil C accumulated over a 21-year period in both 0–20 cm and 20–60 cm depths, and the C accumulation is positively associated with species richness and the abundance of legume and C4 grasses. Despite this extensive research at CCESR, we still lack the understanding of how deeper soil C and N stocks change, as wells as their controlling factors in abandoned agricultural fields, which are undergoing natural reestablishment.
In this study, we repeated soil C and N stock surveys and vegetation sampling of Knops and Bradley (2009). The soil C and N stock dynamics that reported by Knops and Bradley (2009) were based on one time sampling of the old field chronosequence and did not show significant trend in the subsurface soil. With repeated survey, our study directly compare soil C and N stocks changes in the same locations and expect to produce more accurate results. We also measured inputs and outputs of SOM at different depths, including SOM decomposition and DOC. Our objectives were to (1) quantify soil C and N stock changes in both surface and subsurface soil, (2) evaluate the relationship between plant composition, richness, functional group composition and subsurface soil C storage, and (3) discuss the mechanisms controlling subsurface soil C and N stock changes.