Responses of key carbon cycling processes to the addition of water and fertilizers to sandy grassland in semi-arid region

In order to find out the main factors limiting further natural restoration of grassland productivity in the southeastern Keerqin Sandy Lands, the paper analyzed the effects of manipulation of water (80 mm), nitrogen (N, 20 g/(m²·a)) and phosphorus (P₂O₅, 10 g/(m²·a)) on the aboveground net primary productivity (ANPP) and soil respiration of sandy grassland in a fully factorial experiment. In total, there were eight treatments: water addition (W), nitrogen fertilizer addition (N), phosphorus fertilizer addition (P), water + nitrogen fertilizer addition (WN), water + phosphorus fertilizer addition (WP), nitrogen + phosphorus fertilizer addition (NP), water + nitrogen + phosphorus fertilizer addition (WNP) and control (CK). Each treatment was replicated six times and randomly assigned to 48 plots (4 m×4 m) which were separated by a 2 m buffer.Resultsshowed that nitrogen fertilizer not only increased ANPP but also delayed the growth peak. It was found that only nitrogen limited further restoration of productivity and this ecosystem was not limited by water and phosphorus. Soil respiration was most active in July and the difference between adding water and non-adding water treatments was significant. Water addition increased soil CO₂ efflux, whereas phosphorus addition restrained soil respiration. The results suggest that reasonable management of sandy grassland should include no irrigating, adequate applications of nitrogen fertilizers and phosphorus fertilizers in this semiarid region.