Changes of Nitrogen Storage in Soil under Different Land Uses in the Sanjiang Plain

The changes of nitrogen(N) content and its storage in soils under different land uses in the Sanjiang Plain were analyzed to explore the characteristic of the evolution of N storage.The results showed that the total nitrogen(TN) content and its storage in wetland soil were higher than those in fire wetland and desertion cropland soils.The reclamation depleted soil N content,and the TN content and its storage in soils showed a decreasing trend as the increase of cultivation years,which accorded with first-order exponential decay model(P0.01).The sequential fire for 5 years induced the TN content and its storage in soils to be decreased significantly,while the desertion under "Grain for Green" Project for 7 years induced the values increased,but the increase was very limited,which indicated that the soil N pool,once depleted,was hard to restore.The TN content in soil was found to be positively related to organic matter(P0.01),suggesting that the decrease of organic matter due to reclamation might be the main cause of the decrease in N fertility of the soils.So that,increasing soil organic matter is propitious to the maintenance and increase of soil N fertility.The moderate anthropogenic N(such as agricultural drainage) was advised to be imported into the fire wetland,which could restrain the sustaining loss of N in soil and prevent the degradation of wetland vegetations.In addition,the adverse effects caused by agriculture drainage that imported into rivers directly also could be avoided.