Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils

Abstract

The ratio of soil organic carbon stock (SOC) to annual carbon input gives an estimate of the mean residence time of organic carbon that enters the soil (MRTOC). It indicates how efficiently biomass can be transformed into SOC, which is of particular relevance for mitigating climate change by means of SOC storage. There have been few comprehensive studies of MRTOC and their drivers, and these have mainly been restricted to the global scale, on which climatic drivers dominate. This study used the unique combination of regional‐scale cropland and grassland topsoil (0–30 cm) SOC stock data and average site‐specific OC input data derived from the German Agricultural Soil Inventory to elucidate the main drivers of MRTOC. Explanatory variables related to OC input composition and other soil‐forming factors were used to explain the variability in MRTOC by means of a machine‐learning approach. On average, OC entering German agricultural topsoils had an MRT of 21.5 ± 11.6 years, with grasslands (29.0 ± 11.2 years, n = 465) having significantly higher MRTOC than croplands (19.4 ± 10.7, n = 1635). This was explained by the higher proportion of root‐derived OC inputs in grassland soils, which was the most important variable for explaining MRTOC variability at a regional scale. Soil properties such as clay content, soil group, C:N ratio and groundwater level were also important, indicating that MRTOC is driven by a combination of site properties and OC input composition. However, the great importance of root‐derived OC inputs indicated that MRTOC can be actively managed, with maximization of root biomass input to the soil being a straightforward means to extend the time that assimilated C remains in the soil and consequently also increase SOC stocks.

Low N2O emissions induced by root-derived residues compared to aboveground residues of red clover or grasses mixed in the soil hinge on slow decomposition rate

“…The feeble contribution of BGR to soil respiration compared to that of incorporated AGR indicates that the formation of anaerobic conditions and the activity of heterotrophic denitrifiers was limited by lack of sufficiently labile substrate, thus explaining the low N2O emissions (EFN2O < 0.09%, SE ⪝ 0.06) even in the presence of high NO3concentration in the soil, similarly to what was observed in the fallow treatments. A noticeably lower apparent decomposition rate of in situ BGR versus AGR incorporated in the soil is in agreement with previous studies (e.g., Balesdent and Balabane, 1996;Kätterer et al, 2011;Poeplau et al, 2021;Puget and Drinkwater, 2001;Rasse el al., 2005) and was also reflected in the low net mineralization rate (both leys ~1.15 g m -2 in the first batch, i.e., 7% and 5% of N in BGR of grass and red clover, respectively).…”

“…They attributed this to both physical and chemical protection by soil particles attached to the root surface and microaggregates around fine roots, as roots establish close contact with soil particles during growth. The clayey texture of our soil probably contributed to slow down the decay rate, as there is evidence that the large specific surface area and tendency to aggregate, typical for soils with a high clay content, delays the decomposition of roots and AGR (Thomsen et al 1996;Abiven et al 2005;Frøseth and Bleken 2015;Poeplau et al 2021).…”

The potential of cover crops to increase soil organic carbon storage in German croplands

“…This illustrates the value of model ensembles compared to single models when they proved to better describe managementinduced SOC trends in that region (Riggers et al 2019). In addition, Taghizadeh-Toosi and Olesen (2016) did not differentiate between aboveground and belowground C input, even though roots and their exudates contribute more effectively to the stable SOC pool (Kätterer et al 2011;Poeplau et al 2021;Taghizadeh-Toosi and Olesen 2016). In our approach, the different aboveground and belowground C inputs and their effectiveness were taken into account with different partition coefficients introduced by Dechow et al (2019) for RothC.…”

Beyond bulk: Density fractions explain heterogeneity in global soil carbon abundance and persistence

“…At present, belowground NPP, root density, and root biomass metrics are not well quantified at large spatial scales. Furthermore, increasing our understanding of soil organic C origin (e.g., root vs. shoot) and composition (Hall et al, 2020) should increase our understanding of the role of C inputs in regulating turnover (Poeplau et al, 2021).…”