Storage of soil carbon as particulate and mineral associated organic matter in irrigated woody perennial crops

Abstract

Agricultural practices such as annual crop production, land use change and grazing on marginal lands lead to a loss of soil carbon (C) stock. But soil C losses are not universal in agricultural systems and modest soil C gains can occur when constraints such as a lack of water are removed. To characterize this we used a meta-analysis of published data focused on semi-arid regions, where irrigation is required for crop production. We showed that soil C stocks declined under cereals, cotton, maize and non-woody horticultural crops when compared to native unirrigated adjacent grassland or shrubland. By contrast, cultivation of irrigated, woody perennial crops generally leads to an accumulation of soil C. Identifying the mechanisms by which C is retained in the soils beneath woody perennial crops, and any limits to C accumulation, was the main goal of this study. A mechanistic understanding of soil organic C content accumulation, upon land use change, can be gained by dividing soil into particulate organic matter (POM) and mineral associated organic matter (MAOM). Here, we analyzed the C and natural abundance 13C concentrations in POM and MAOM fractions in soils from eight apple orchards and eight vineyards irrigated using a dripline, and eight apple and eight cherry orchards irrigated with micro-spray. Samples were also taken from eight native grassland areas adjacent to the agricultural sites for comparison. Several decades of woody crop production doubled the average soil C concentration in comparison to the native sites, from to 10.1 ± 1.48 g C kg−1 to 20.1 ± 0.96 g C kg−1 over a depth of 0–15 cm. Most of the C was associated with POM, which increased in concentration from 7.9 ± 1.19 g C kg−1 to 14.2 ± 0.79 g C kg−1 in 0–15 cm soils, an increase of 80%. This was crop dependent, being highest in the cherry orchards and lowest in the vineyard soils. Although holding less C, the MAOM concentration increased by 166%, changing from 2.22 ± 0.33 to 5.91 ± 0.62 g C kg−1; no differences existed between crops but the MAOM C concentration appeared to be constrained to a maximum value of ~ 12 g C kg−1. MAOM and POM had markedly different δ13C values: MAOM was more enriched, indicative of greater microbial processing, whereas POM had a lower δ13C value consistent with the dominant standing vegetation. δ13C values were more depleted in both fractions at the agricultural sites compared to the native sites, indicating the accumulation of greater amounts of less processed C at the agricultural sites, due to higher C inputs. We conclude the soils in this region respond to irrigated perennial woody crop production by retaining C within both POM and MAOM fractions. Accumulation of C as MAOM is constrained by the minerology of the soils in this region, but is unconstrained for POM which dominates the soil C content, and is potentially vulnerable to changes in management practices and land use.