Nitrogen budgets and environmental capacity in farm systems in a large-scale karst region, southern China

Field surplus nitrogen (N) and farm disposal N are major sources of water pollution in farming systems. These sources are estimated from N budgets in field and whole farms, which are associated with the production and consumption of food. This study was conducted to evaluate these two pollution sources in the steep mountainous karst region of Quibainong, Guangxi Province, southern China. The region is, characterized as an area of upland farms, due to the shallow soils and rapid water drainage through cracks in the limestone. Although field surplus N in 1960 was only 4.1 kg N ha(-1), current field surplus N ranged from 10.1 to 463 kg N ha(-1), with values above 50 kg N ha(-1) in farms along roads and less than 40 kg N ha(-1) in the farms away from roads. The results obtained in near-road farms were similar to those in a previous study of N budgets in China. There was a significant positive correlation between the field surplus N and N application rate, including when the previous data were incorporated. The proportion of manure to total N application decreased with increase of N application. Chemical fertilizer was applied in greater quantity in economically rich farms. Therefore, the increase of field surplus N in Quibainong may be caused by economic improvement. Although livestock and human excreta were stocked in manure barns, unused excreta N increased with the increase of N excreted. The unused excreta N also increased with the decrease of feed self-sufficiency, but was not related to N application rate. These facts indicate that livestock husbandry in Quibainong is related to economic status of farms, but independently of crop production. The N application rate of more than 160 kg N ha(-1) increased field surplus N to an extent greater than crop uptake N, and a N application rate of more than 185 kg N ha(-1) increased the potential nitrate-N concentration to more than 10 mg L-1. Therefore, 160-185 kg N ha(-1) is suggested to be the environmental capacity to sustain optimal N cycling in Quibainong. The average value of excreta N produced on near-road farms in Quibainong was 171 kg N ha(-1). If excreta N was used evenly for crop cultivation without chemical fertilizer in whole fields, the optimal N cycling would be maintained. The survey conducted here using a questionnaire was effective in evaluating all kind of N flows in the farming systems.