Organic Carbon Mineralization Characteristics Of Paddy Soil In Nanniwan With Different Planting Years Under The Background Of Temperature Changes And Nitrogen Deposition

In order to explore the response of organic carbon mineralization in paddy soil with different planting years to temperature changes and nitrogen addition,the rice field soil samples with different planting years（3 a,30 a,78 a）were collected at the Nanniwan Rice Planting Base in the tapically loess hilly and gully regions of northern Shaanxi provience.The characteristics and differences of organic carbon mineralization were measured through indoor 35 day cultivation experiments and mineralization dynamic models between paddy and dry land soils.Temperature changes and nitrogen deposition had a significant impact on organic carbon mineralization.The main results were as follows:（1）The cumulative mineralization amount and mineralization rate of soil organic carbon under different tillage years were shown as 78 a>30 a>3 a>maize soil.The cumulative mineralization amount,mineralization rate,potential mineralizable organic carbon content（C₀）and the ratio of potential mineralizable organic carbon to OC（C₀/SOC）were also increased with the improvement of temperature.Under various cultivation temperature conditions,the average mineralization accumulation in paddy soil was higher than that in maize soil,with increasing of 31.65%.The accumulation of organic carbon mineralization in paddy soil showed a significant decrease at 10℃compared to 20℃and 30℃（P<0.05）,but there is no significant difference between20℃and 30℃.The accumulated mineralization of organic carbon in the 0-20 cm soil layer was higher than that in the 20-40 cm soil layer.The mineralization rate reached its maximum on the 1st day of cultivation,but the average mineralization rates were decreased by 36.91%,15.29%,and 10.82%on the 7th,21st,and 35th days when compared to the 1st day,respectively.On the 14th day of the maize field,the average decreases in soil microbial biomass carbon（MBC）and dissolved organic carbon（DOC）were recorded at 24.19%and 19.16%,while the average decreases in MBC and DOC were recorded at 37.39%and 39.13%on the 35th day,The C₀/SOC values decreased with the improve of rice planting years,indicating that the soil organic carbon sequestration capacity be enhanced with the rice planting years.The potential mineralizable organic carbon content C₀ had a significant positive correlation with TOC,easily oxidized organic carbon and dissolved organic carbon.The relationship between C₀ and microbial biomass carbon indicated a positive correlation,but not reached a significant level.The temperature coefficient(Q₁₀)was higher under temperature increased from 10℃to 20℃when relative to the temperature increased from 20℃to 30℃,and the order of Q₁₀ values under different layers as follows:0-20 cm>20-40cm.（2）The cumulative mineralization amount with constant temperature and periodic temperature changes was rapidly increased in 0-7 days,reaching to 39.2%-47.6%of the entire cultivation period,and slowly increased after 7th day.The cumulative mineralization amount was higher in 0-20 cm layer than that in 20-40 cm layer.It reached the peak values at the 1st day then decreased immediately,but stabilized in a status with the extension of cultivation time.Before 7 days of cultivation,the mineralization rate of soil organic carbon rapidly decreased,and then gradually decreased until it reached a stable status at 21 days of cultivation.The mineralization rates of organic carbon in the 0-20 cm were significantly higher than that in 20-40 cm.The cumulative mineralization amount and mineralization rate of soil organic carbon under different tillage years were shown as 78 a>30 a>3 a>maize soil.Under periodic temperature changes,the decreased values in DOC and MBC of maize fields after 14days were higher than that of constant temperature cultivation,and the decreased values in MBC and DOC after 35 days were lower than that of constant temperature cultivation.The decreased values in MBC and DOC of paddy soil under periodic temperature changes at 14 days were higher than that of constant temperature cultivation,while the opposite trend were recorded after 35 days.The correlation between soil C₀ and organic carbon mineralization constant（k）and soil properties under periodic variable temperature conditions were relatively higher than that under constant temperature conditions.The significant difference was not been recorded in soil organic carbon mineralization rate,cumulative mineralization amount,and potential mineralizable organic carbon content between variable temperatures and constant temperatures,indicating that periodic temperature changes had no significant impact on soil organic carbon mineralization under the same accumulated temperature.（3）The mineralization of soil organic carbon was inhibited in paddy soil when the ammonium nitrogen addition with 150 mg/kg,leading to a decrease trend in mineralization accumulation,mineralization rate,and mineralization rate.Compared with0 mg/kg ammonium nitrogen addation,the organic carbon mineralization was increased in 3 a paddy soils,78 a paddy soils and maize fields under 100 mg/kg and 50 mg/kg ammonium nitrogen addition,,while the inhibited trend was measured in 30 a paddy soils.Nitrate nitrogen at 50 mg/kg,100 mg/kg,and 150 mg/kg only had an inhibitory effect on the accumulation of mineralization and mineralization rate.Under the same concentration,nitrate nitrogen had a significant inhibitory effect on organic carbon mineralization than ammonium nitrogen.The cumulative mineralization amount and mineralization rate of soil organic carbon under different tillage years were shown as 78a>30 a>3 a>maize soil.The cumulative mineralization rate showd an increased trend and then decreasing with the increase of ammonium nitrogen addition level.The cumulative mineralization amount and mineralization rate of 0-20 cm were significantly higher than that in 20-40 cm（P<0.05）.For the 0-20 cm soil layer,the average values in MBC and DOC were decreased by 30.70%and 34.50%on the 14th day,while that were decreased by 54.63%and 56.80%on the 35th day;The values were decreased by 31.69%and 34.67%in MBC and DOC on the 14th day of the 20-40 cm soil layer,while the decreased values were 42.21%and 55.27%on the 35th day,respectively.Overall,the decreased value of MBC in the 0-20 cm soil layer was lower than that in the 20-40 cm soil layer at 14 days,while it showed the opposite trend in 20-40 cm soil layer;At 35days,the decreased values in MBC and DOC in the 0-20 cm soil layer were higher than that in the 20-40 cm soil layer.The relationship between C₀ and k values and soil properties showed no significant correlation with the increase of nitrogen addition.