Phosphorus enrichment helps increase soil carbon mineralization in vegetation along an urban-to-rural gradient,Nanchang, China

We used four vegetation types located along an urban–suburban–rural gradient in Nanchang, China to study how the deposition of nitrogen (N) and phosphorus (P) in the urban area affected soil carbon (C) cycling. We found that total P, nitrate (NO₃^?–N), available P, and the abundances of culturable bacteria, actinobacteria, and nitrifying bacteria in soils, collected to 15 cm depth in August of 2008, decreased along the urban-to-rural gradient (P < 0.05); the C/P and N/P ratios, ammonium (NH₄⁺–N), and culturable fungi abundance showed the reverse trends; whereas soil organic C, total N, C/N, mineral N, and the activities of sucrase and neutraland acid phosphatase showed no pattern with gradient and vegetation type. Compared to suburban and rural sites, total and available P in soil increased 168% and 131%, 47% and 139%, respectively in urban sites. The cumulative amount of CO₂ emission per gram of soil (C_min, incubated from 2 to 43 days) varied little along the urban-to-rural gradient, but showed positive correlations with organic C, total N, total P, nitrate, mineral N concentrations, C/N, bacteria and actinobacteria abundances, sucrase and acid phosphatase activities. In contrast, the cumulative amount of CO₂ produced per gram organic C (C_min/OC) within the incubation period was influenced by gradient, vegetation type, and their interaction, and values were about 35% greater in the urban than in suburban and rural sites. The relationship between elevated C_min/OC in urban vegetations and the enrichment of P in organic matter (P/C ratio) suggests that P coming from urban household waste can degrade the stability of organic C in urban soils.