Minghua Zhou

Nitrate leaching, direct and indirect nitrous oxide fluxes from sloping cropland in the purple soil area, southwestern China

This study provides a combined dataset on N loss pathways and fluxes from sloping cropland in the purple soil area, southwestern China. A lysimeter experiment was conducted to quantify nitrate leaching (May 2004-May 2010) and N(2)O emission (May 2009-May 2010) losses. Nitrate leaching was the dominant N loss pathway and annual leaching fluxes ranged from 19.2 to 53.4 kg N ha(-1), with significant differences between individual observation years (P < 0.05). Direct N(2)O emissions due to N fertilizer use were 1.72 ± 0.34 kg N ha(-1) yr(-1), which corresponds to an emission factor of 0.58 ± 0.12%. However, indirect N(2)O emissions caused by nitrate leaching and surface runoff N losses, may contribute another 0.15-0.42 kg N ha(-1) yr(-1). Our study shows that nitrate leaching lowered direct N(2)O emissions, highlighting the importance for a better understanding of the tradeoff between direct and indirect N(2)O emissions for the development of meaningful N(2)O emission strategies.

Regional nitrogen budget of the Lake Victoria Basin, East Africa: Syntheses, uncertainties and perspectives

Using the net anthropogenic nitrogen input (NANI) approach we estimated the N budget for the Lake Victoria Basin in East Africa. The NANI of the basin ranged from 887 to 3008 kg N km �2 yr �1 (mean: 1827 kg N km �2 yr �1 ) for the period 1995–2000. The net nitrogen release at basin level is due primarily to livestock and human consumption of feed and foods, contributing between 69% and 85%. Atmospheric oxidized N deposition contributed approximately 14% to the NANI of the Lake Victoria Basin, while either synthetic N fertilizer imports or biological N fixations only contributed less than 6% to the regional NANI. Due to the low N imports of feed and food products (<20 kg N km �2 yr �1 ), nitrogen release to the watershed must be derived from the mining of soil N stocks. The fraction of riverine N export to Lake Victoria accounted for 16%, which is much lower than for watersheds located in Europe and USA (25%). A significant reduction of the uncertainty of our N budget estimate for Lake Victoria Basin would be possible if better data on livestock systems and riverine N export were available. Our study indicates that at present soil N mining is the main source of nitrogen in the Lake Victoria Basin. Thus, sustainable N management requires increasing agricultural N inputs to guarantee food security and rehabilitation and protection of soils to minimize environmental costs. Moreover, to reduce N pollution of the lake, improving management of human and animal wastes needs to be carefully considered in future. S Online supplementary data available from stacks.iop.org/ERL/9/105009/mmedia

Carbon budget and greenhouse gas balance during the initial years after rice paddy conversion to vegetable cultivation

Rice paddy conversion to vegetable production is a common agricultural practice driven by economic benefits and shifting diets. However, little is known on the initial effects of this land-use conversion on net ecosystem carbon budget (NECB) and greenhouse gas (GHG) balance. Annual NECB and emissions of CH4 and N2O were measured from a native double rice cropping system (Rice) and a vegetable field recently converted from rice paddy (Veg) under no nitrogen (N) fertilization (Rice-N0 and Veg-N0) and conventional N fertilization (Rice-N+ and Veg-N+) during the initial four years upon conversion in subtropical China. Land-use conversion from rice to vegetable cultivation led to substantial C losses (2.6 to 4.5 Mg C ha-1 yr-1), resulting from strongly reduced C input by 44-52% and increased soil organic matter mineralization by 46-59% relative to Rice. The magnitude of C losses from Veg was highest in the first year upon conversion, and showed a decreasing trend over time. N fertilization shifted rice paddy from a slight C source in Rice-N0 (-1.0 Mg C ha-1 yr-1) to a significant C sink in Rice-N+ (1.1 Mg C ha-1 yr-1) and alleviated the impact of land-use conversion on C loss via increased C input from higher crop productivity. Land-use conversion greatly increased the global warming potential (GWP) from Veg by 116-395% relative to Rice in the first year, primarily due to increased C losses and N2O emission outweighing the decreased CH4 emission. However, the GWP did not show obvious difference between Rice and Veg in the following years. N fertilization and land-use conversion interactively increased GWP in the first year via increased N2O production. Concluding, NECB and GHG emissions in the first year after conversion are crucial and should be considered when evaluating the environmental consequences of land-use conversion.