Mitigation of nitrogen and phosphorus leaching from cropland in northern China

Understanding on nitrogen (N) and phosphorus (P) leaching from root-vadose-groundwater system and mitigation options is research gaps in the world. We presented the main research progresses of a project 'Mitigation of Nitrogen and Phosphorus Leaching from Cropland in China' in National Key Research and Development Program in this paper. Four research contents are included: 1) clarifying temporal and spatial variation of N and P leaching out of root zone; 2) analyzing nitrogen and phosphorus leaching in the 'root-vadose-groundwater' system; 3) exploring mitigation options of leaching in cereals and vegetables fields of three soil types, namely black soil, cinnamon soil and fluvo-aquic soil; and 4) exploring regional strategies for decreasing N and P leaching in the 'root-vadose-groundwater' system. Main scientific findings are as follows: 1) the N leaching in root zone shows different trends with exceeding standard rates of nitrate in groundwater in three soil type areas, due to their differences in land use, groundwater depth, lithologies of vadose zones and hydrogeological conditions etc. In black soil areas, although the N leaching is not high in root zones, the interactions between groundwater quality and N leaching in root zone are more sensitive because of the topography. Therefore, more researches are needed to explore the interaction mechanism between groundwater quality and leaching N in black soil areas. In cinnamon and fluvo-aquic soil areas in the North China Plain, vadose zones are deep and can buffer N leaching from root zone to groundwater. Thus, it is also necessary to further explore the N leaching mitigation mechanism of vadose zones. 2) Based on the long-term fertilization experiments and observation in 12 m deep borehole, we analyzed the accumulation characteristics of N surplus in vadose zones of farmland. The results show that the safe N application rate in the North China Plain is about 200 kg(N)·hm^-2·a^-1. If the threshold is exceeded, 51% of the N would leach to out of the root areas (1 m). The unreasonable irrigation, heavy rainfall, macrovoids and crack are the main causes of soil nitrate leaching. It can lead accumulated nitrate in vadose zone to be leached to below 6 m. 3) Combined deep sampling and biological method, we analyzed denitrification activity and floristic composition of soil microbe in 0-10.5 m of vadose zones. The results show that surface soil is main site for microbial denitrification, while in deep soil layer the denitrification weaken significantly, which indicates that "Carbon Starvation" is the key factor for limiting the abundance and activity of denitrifying microbes in the bottom soil. Furthermore, the indoor incubation experiment proved that adding carbon could effectively activate the soil denitrifying microbes, which explained the mechanism of "the nitrogen interception in root areas and denitrification in vadose zones". 4) Using the data from N and P leaching mitigation experiments, national agricultural non-point source pollution monitoring network, groundwater nitrate monitoring network in northern China and NUFER (NUtrient flows in Food chains, Environment and Resources use) model, we compartmented nutrient losses vulnerable zones. Based on this, we explored regional mitigation strategies of N and P leaching, which could provide scientific support for non-point sources pollution.