Nitrogen addition stimulates soil aggregation and enhances carbon storage in terrestrial ecosystems of China: A meta-analysis

China is experiencing a high level of
atmospheric nitrogen (N) deposition, which
greatly affects the soil carbon (C) dynamics in terrestrial ecosystems. Soil aggregation contributes to the stability
of soil structure and to soil C sequestration. Although many studies have reported the effects
of N enrichment on bulk soil C dynamics, the underlying mechanisms explaining
how soil aggregates respond to N enrichment remain unclear. Here, we
used a meta-analysis of data from 76 N manipulation experiments in terrestrial
ecosystems in China to assess the effects of N enrichment on soil aggregation and
its sequestration of C. On average, N
enrichment significantly increased the mean weight diameter of soil aggregates
by 10%. The
proportion of macroaggregates and silt-clay fraction were significantly
increased (6%) and decreased (9%) by N enrichment, respectively. A greater response of macroaggregate C (+15%)
than of bulk soil C (+5%) to N enrichment was detected across all ecosystems. However,
N enrichment had minor effects on microaggregate C and silt-clay C. Magnitude of N enrichment effect on soil aggregation
varied with ecosystem type and fertilization regime. Additionally, soil pH declined consistently
and was correlated with soil aggregate C. Overall, our meta-analysis suggests
that N enrichment promotes particulate organic C accumulation via increasing macroaggregate
C and acidifying soils. In contrast, increases in soil aggregation
could inhibit microbially-mediated breakdown of soil organic matter, causing
minimal change in mineral-associated organic C. Our
findings highlight that atmospheric N deposition may enhance the formation of
soil aggregates and their sequestration of C in terrestrial ecosystems in
China.