Comprehensive evaluation for nitrogen footprint and gray water footprint of agricultural land use system

Abstract: Footprint research has gained more and more attention in ecology and sustainable development research fields. Nitrogen (N) footprint has theoretical and practical significance in evaluating the effect of activities on reactive N emission. Grey water footprint (GWF) is an indicator of aquatic pollution, which is important in the assessment of environmental effects of pollutants on water resource. In this paper, we employed both N footprint and GWF to evaluate environmental effects of agricultural land use system on reactive N loss and water quality. Based on the framework of the N footprint and the GWF, we built up an indicator system and assessed the environmental effects of agricultural land use system at the regional scale. To test the approach, the Taojiang County was chosen as the case study area. We assessed the N footprint and the GWF by using the official statistics during 1980 and 2010. The N footprints and the GWFs of the past agricultural land use system showed a large variation from 1980 to 2010 in Taojiang County: the total N footprint and the GWF increased drastically by 102.18% and 136.42% (i.e., from 15135.09 to 30599.40 t, and from 4.35×108 to 10.27×108 m3) respectively during the period of 1980-2010. The unit area N footprint and GWF were 0.35 t/hm2, 1.01×104 m3/hm2 in 1980 and increased to 0.70 t/hm2, 2.33×104 m3/hm2 in 2010. The input N footprint was increased from 15 084.23 t in 1980 to 30 551.39 t in 2010, while the unit area input N footprint were 0.35 t/hm2 in 1980 and then increased to 0.69 t/hm2 in 2010. The results indicated that the growth of fertilizer input was the main reason for N footprint increasing, which accounted for 72.72% of the input N footprint in 2010. Compared with 1980, the pollution N footprint gradually increased to 10 032.79 t in 2010. The proportion of pollution N footprint reached 32.84% in 2010. The unit area pollution N footprint was 0.10 t/hm2 in 1980 and increased to 0.23 t/hm2 in 2010. Based on the results, the GWF of N fertilizer was higher than that of phosphate fertilizer; and we selected the GWF of nitrogen fertilizer as the total GWF of agricultural land use system in Taojiang County. Compared with 1980, the total GWF of agricultural land use system in Taojiang County gradually increased to 10.27×108 m3 in 2010. The GWF of phosphate fertilizer increased from 1.88×108 to 5.92×108 m3 between 1980 and 2010. The unit area GWF of phosphate fertilizer was 0.44×104 m3/hm2 in 1980 and increased to 1.35×104 m3/hm2 in 2010. Increasing virtual water that was used to dilute the growing loss of reactive N was the key factor that had caused the increase of total GWF in Taojiang County. The result indicated that agricultural land use system had become the main contributor of air pollution and non-point source pollution, and this comprehensive approach in this paper could assess the environmental negative effects of agricultural land use system effectively. Based on the method, the negative effects of environmental issues on the N footprint, the GWF in county level were quantified preferably. The environmental sustainability of agricultural land use system was facing severe challenges in the increasing N footprint and the GWF of agricultural land use system in Taojiang County. The measures, including optimizing field management technique such as N fertilizer reduction, agricultural land use system designing, land use structure adjustment, are recommended for reducing the N footprint, the GWF and their environmental risk. The method proposed in this paper can provide a novel approach to quantify the losses of reactive N to the environment and to evaluate the environmental effects of agricultural land use system on water pollution level. The results can offer a valuable reference for the agricultural sustainable development and land use system optimal utilization for other similar grain producing regions.