Responses of farmland soil organic carbon to key natural and landscape factors: Threshold effects and nonlinearity

Abstract

Exploring the nonlinear relationships and impact thresholds between farmland soil organic carbon (SOC) and key environmental factors will help formulate SOC sequestration policies. Taking a watershed in Changtu County, an important grain-producing area in northeast China, as the study area, we determined the key natural and landscape factors, and the thresholds of single-factor and multi-factor interactions on SOC. The results showed that precipitation, elevation, and temperature in natural factors; proportions of forestland, construction land, and water bodies in landscape composition; largest patch index (LPI_FL) and edge density (ED_FL) of forestland; and LPI of construction land (LPI_CL) in landscape configuration were key factors. Thresholds for SOC were observed for each factor except for water body. Moreover, we found threshold interactions between multiple landscape composition and configuration factors. Forestland and construction land were the main landscape types driving SOC. Specifically, in landscape composition, when construction land accounted for ≤ 11.419%, ensuring forestland exceeded 5.536% could alleviate SOC loss. When construction land was > 11.419% or 19.464%, SOC showed a downward trend that was difficult to reverse. In landscape configuration, increasing LPI_FL and ED_FL had positive effects, whereas increasing LPI_FL to maintain SOC required controlling LPI_CL to within 5.363 or 10.470; otherwise, the benefit was limited. In summary, we suggest that proportion or patch area of construction land should be controlled within a threshold range, beyond which SOC loss will exceed the compensatory capacity of forestland. The quantified thresholds are valuable tools for providing guidance for implementing landscape optimization aimed at SOC sequestration.