The patterns of soil nitrogen stocks and C  :  N stoichiometry under impervious surfaces in China

Abstract

Accurate assessment of soil nitrogen (N) storage and carbon (C) : N stoichiometry under impervious surface areas (ISAs) is key to understanding the impact of urbanization on soil health and the N cycle. Based on 888 soil profiles from 148 sampling sites in 41 cities across China, we estimated the country's N stock (100 cm depth) in the ISA soil to be 98.74±59.13 Tg N with a mean N density (NISA) of 0.59±0.35 kg m−2, which was significantly lower (at all depths) than the soil N density (NPSA=0.83±0.46 kg m−2) under the reference permeable surface areas (PSAs). The NISA was also only about 53 %–69 % of the reported national mean soil N density, indicating that ISA expansion caused soil N loss. The C:N ratio of ISA (10.33±2.62) was 26 %–34 % higher than that of natural ecosystems (forests, grasslands, etc.) but close to the C:N of PSA. Moreover, there was a significant C–N correlation in ISA soil, showing no signs of C–N decoupling as suggested by the previous studies. The ISA had smaller variances in the C:N ratio than did the PSA at regional scale, indicating convergence of soil C:N stoichiometry due to ISA conversion. The eastern subregion of China had the highest NISA, although its natural soil N density was among the lowest in the country. Unlike the vertical pattern in natural permeable soils, whose N density declined faster in the upper soil layers than in the lower layers, NISA decreased linearly with depth. Similarly to natural soil N, NISA was negatively correlated with temperature; but unlike natural soil C:N which was positively correlated with temperature, the C:NISA was negatively correlated with temperature. NISA was not correlated with net primary productivity, but was significantly correlated with the soil N density of adjacent PSA and the urbanization rate. These findings indicate the ISA soil had a unique N distribution pattern, possibly as the result of intensive disturbances during land conversion. The dataset “Observations of soil nitrogen and soil organic carbon to soil nitrogen stoichiometry under the impervious surfaces areas (ISA) of China” is available from the National Cryosphere Desert Data Center (https://doi.org/10.12072/ncdc.socn.db2851.2023) (Ding et al., 2023).