Microbial responses to the erosional redistribution of soil organic carbon in arable fields

Abstract

Quantifying the potential for eroding agricultural soils to act as sinks or sources of atmospheric carbon relies on accounting for the pools and fluxes of soil organic carbon (SOC) and nutrients, e.g. nitrogen (N), affected by erosion. Herein, we report the outcomes of an experiment where a C4 maize (Zea mays) crop (δ13C = −12.1‰) was cultivated and incorporated for 2 years to introduce a ‘pulse’ of 13C-enriched SOC to a C3 arable soil (δ13C = −27.4‰). Soils were sampled at eroding (top slope and upper slope) and depositional (lower slope and slope foot) positions of an accelerated erosion pathway that were confirmed using 137Cs measurements. The sand particle-sized fraction (63–2000 μm) was predominant and increased in the depositional slope positions due to selective loss of fine particles and preferential deposition of the coarsest fraction of transported sediment. There was a significant isometric relationship between the percentage SOC and total N: top slope > upper slope > lower slope, with similar values in the slope foot to the top slope. The δ15N values of the soils were enriched (7.3‰) at the slope foot, compared with the other slope positions (average 6.3‰), suggesting increased denitrification rates. The δ13C values of the soil microbial biomass C extracted from surface soils (0–5 cm) at each slope position showed that the proportion of maize C being incorporated into the soil microbial biomass declined in the downslope direction from 54% (top slope) to 43% (upper slope) to 18% (lower slope) in inverse proportion to the size of the soil microbial biomass, and increased to 41% at the slope foot. This suggests dynamic replacement of the SOC with crop C in the eroding slope positions and dilution of the transported C by C3-SOC in the depositional slope positions. This paper is evidence that erosional distribution of soil carbon leads to differential microbial utilisation of SOC between eroding and depositional sites.