Soil erosion significantly decreases aggregate-associated OC and N in agricultural soils of Northeast China

Cultivation after the converting natural ecosystems to agricultural land is often accompanied by accelerated soil erosion. To better understand how land-use conversion and agricultural erosion separately affect aggregateassociated nitrogen (N) and organic carbon (OC), we examined the responses of aggregate-associated N and OC to both the converting forest to agricultural land and erosion in the agricultural region of Northeast China. Non-erosion sites in forest and agricultural land converted from forestland 60 years ago were selected to address the effects of land-use change. The sites with various erosion intensities in an eroding area of sloping cropland (i. e., non-erosion, light erosion, moderate erosion and heavy erosion) were selected to compare the effects of the erosion intensity. The measured soil metrics included the soil bulk density, water-stable aggregates, and aggregate-associated OC and N along 0-70 cm soil profiles. Converting forest to agricultural land did not significantly influence the OC and N concentrations in each aggregate, while the land-use change significantly reduced the OC and N stocks in macroaggregates but increased them in microaggregates and silt+clay fraction. However, erosion significantly decreased the OC and N concentrations and stocks in each aggregate. The changes in the OC and N stocks in aggregates were mainly due to the depletion of soil aggregates in responding to the conversion of forest to agricultural land but due to decreases in OC and N concentrations in aggregates in responding to soil erosion. Our results indicated that the effects of erosion on aggregate-associated OC and N were greater compared with the effects of converting forest to agricultural land in this Mollisol region.

Automated summary

The study found that converting forest to agricultural land had minimal impact on OC and N concentrations in soil aggregates, but significantly affected the stocks of OC and N in the aggregates. In contrast, erosion had a significant negative impact on both the concentrations and stocks of OC and N in the soil aggregates.