Effects Of Temperature And Moisture On Nitrogen And Phosphorus Conversion In Cambisol And Ultisol

Nitrogen（N）and phosphorus（P）are essential nutrient elements for crop growth.Appropriate soil nitrogen and phosphorus levels are beneficial to improve crop yield and quality and ensure the quality and safety of agricultural products.The release of excessive nitrogen and phosphorus in soil will cause water eutrophication and other environmental problems.Global climate change（warming and frequent rainfall）directly or indirectly affects a series of physical,chemical and biological processes in soils,and determines the biogeochemical cycles,bioavailability and environmental behavior of soil nitrogen and phosphorus.In the past,studies on the response of soil N and P to climate change mainly focused on the effects of forest and alpine meadow soil and single factor of temperature or moisture,while there were few studies on the effects of climate change on soil N and P transformation in farmland.Therefore,it is of great significance to investigate the conversion process of nitrogen and phosphorus in farmland soil under the coupled condition of moisture and heat for guiding the scientific management of soil nutrients,improving soil fertility,and ensuring environmental quality and human health.In this study,Cambisol and Ultisol,which were collected from Shanxi and Hunan province,China respectively,were selected as the research objects.Laboratory incubation experiments of 180 days were conducted at three temperatures of 15℃,30℃and 45℃and three moisture conditions of 40%,60%and 80%WHC（water holding capacity）.Modern analytical techniques such as conventional chemical,biochemical methods and synchrotron X-ray absorption near-edge structures（XANES）have been used to analyzed the contents of available N and P,the rate of N mineralization,the distribution of phosphorus forms in different soil and the enzyme activities related to N and P cycling to understand the effects of temperature and moisture changes on the nitrogen and phosphorus cycling in soil.The main results are as below:（1）Warming significantly increased the content of NH₄⁺-N and the rate of N mineralization in the soil.The response of NO₃^--N in the two soils to warming was different,and the effect of moisture on N mineralization was different at different temperatures.With the increase of temperature,the content of NH₄⁺-N in Cambisol and Ultisol increased by1.6 to 1.9 times and 7.9 to 8.7 times,respectively,and the rate of nitrogen mineralization increased by 5.3 to 8 times and 1.5 to 6 times.The NO₃^--N content in Cambisol increased by 0.3-8.3 times,while the nitrate nitrogen content in Ultisol increased first and then decreased.The increase of moisture had no significant effect on soil inorganic nitrogen content.With the increase of moisture,the soil nitrogen mineralization rate firstly decreased and then increased at 15℃,significantly decreased by 50%-84%at 30℃,and increased by 0.6-1 times at 45℃.（2）Warming increased the content of available P in Cambisol,but decreased the content of available P in Ultisol,and promoted the formation of Fe-P in soils.The increase of moisture had no significant effect on the content of available phosphorus,but promoted the formation of Fe-P,and inhibited the formation of Ca-P and Al-P in Ultisol.The ratios of Ca-P and Fe-P in Cambisol were 71.0%-79.2%and 10.7%-26.1%.The ratios of Fe-P,Al-P and IHP in Ultisol were 46.1%-84.3%,11.0%-45.8%and 12.4%-21.7%,respectively.With the increase of temperature,the content of available P in Cambisol increased by 12.7%-17.7%,and that in Ultisol decreased by 16.6%-18%,and the proportion of Fe-P in the two soils increased by 27.5%-120%.With the increase of moisture,the ratio of Al-P in Ultisol decreased significantly,but had no significant effect on P form in Cambisol.（3）Warming inhibited the activities of urease,N-acetyl-glucosaminidase（NAG）and acid phosphatase（AP）in Cambisol and Ultisol.Increasing moisture had no significant effect on urease activity,but inhibited the activity of NAG.The response of AP activity to moisture was different at different temperatures.With the increase of temperature,urease activity in Cambisol and Ultisol decreased by 94.9%to 96.5%and 44.4%to 68.9%,respectively,NAG activity decreased by 23.1%-86.5%,and AP activity decreased by28.9%-50.5%.With the increase of moisture,urease activity in Cambisol and Ultisol had no significant change,and NAG activity decreased by 28.1%to 51.1%and 34.3%to 86.4%,respectively.AP activity in Cambisol had no significant change at 15℃and 30℃,but increased first and then decreased in Ultisol.AP activity in Cambisol increased by 50%at45℃,and decreased by 95.5%in Ultisol.（4）Warming affects the availability of nitrogen and phosphorus by changing soil p H and enzyme activity.Soil organic carbon is a significant factor influencing the change of enzyme activity under warming conditions.Multiple regression analysis showed that the availability of nitrogen in Cambisol was mainly affected by p H,and its contribution rate was 39.9%-75.2%,significantly higher than that of enzyme activity.The availability of phosphorus was mainly affected by enzyme activity,and its contribution rate was 37.9%-52.7%.The availability of nitrogen and phosphorus in Ultisol was mainly affected by p H and enzyme activity,and the contribution rate was 65.8%-97.5%.Correlation analysis showed that temperature in Cambisol and Ultisol was negatively correlated with TC,and TC content was positively correlated with activities of urease,NAG and AP,indicating that soil warming caused the loss of soil organic carbon,leading to a significant decrease in soil enzyme activities.