| More than 95% of the nitrogen in the soil exists in organic form.Plants cannot be directly absorbed and utilized,and only converted to effective nitrogen by mineralization can be absorbed and utilized by plants.Therefore,the mineralization rate of soil organic nitrogen is an important indicator to measure the nitrogen supply capacity of soil,and the too fast mineralization rate is an important process of soil nitrogen leaching loss.The speed of mineralization depends not only on the kind and amount of organic nitrogen source,but also on soil type,plant type and regional hydrothermal conditions.The forest-steppe ecotone is characterized by the coexistence of forest vegetation and grassland vegetation.It has many characteristics such as many community types,high species diversity,and weak ecological environment anti-interference ability.Located in the interlaced zone of the eastern forest to the western grassland in the northwestern part of Liaoning Province,the grassland type belongs to warm shrubs,distributed with shrubs?hereinafter referred to as grassland?and plantations?hereinafter referred to as forest land?,annual precipitation and average temperature.Both are higher than temperate grassland types.In order to grasp the soil nitrogen transformation characteristics of different vegetation types and the inhibitory effect of inhibitors on nitrification in the forest steppe,the two vegetation types of grassland and woodland were selected as the object,and the nitrogen isotope labeling and laboratory aerobic culture method were adopted.The MCMC model calculates the net soil nitrogen conversion rate,gross conversion rate,microbial retention rate,and N2 O emission rate for the two vegetation types.The nitrogen nitrification of soil by Dicyandiamide?DCD?was analyzed by the combination of urea?0,10 kg·hm-2,20 kg·hm-2,40 kg·hm-2?with nitrification inhibitors.The results and conclusions are as follows:?1?According to the model calculation,the soil nitrogen mineralization rate and the nitrogen gross mineralization rate of the two types of vegetation in the forest steppe have great differences,and the net mineralization rate is nearly half of the gross mineralization rate,confirming the limitations of net mineralization rates in calculations.The gross nitrogen mineralization rate of grassland soil is 1.07 mg·kg-1·d-1,the net mineralization rate is only 0.52 mg·kg-1·d-1,which is 2 times different;the gross ammoniation rate is 1.08 mg·kg-1·d-1,the net ammoniation rate is-0.51 mg·kg-1·d-1,which is 3 times different;the gross nitrification rate is 1.31 mg·kg-1·d-1,and the net nitrification rate is 1.28 mg·kg-1·d-1.The gross mineralization rate of soil nitrogen was 1.35 mg·kg-1·d-1,the net mineralization rate was 1.07 mg·kg-1·d-1,and the gross ammoniation rate was 1.35 mg·kg-1·d-1.The net ammoniation rate is 0.30 mg·kg-1·d-1;the gross nitrification rate is 1.20 mg·kg-1·d-1,and the net nitrification rate is 1.19 mg·kg-1·d-1.That is to say,in the forest-steppe ecotone,the soil ammonia nitrogenization ability of the forest land is stronger than that of the grassland soil,and the soil nitrogen nitrification ability of the grassland soil is stronger than that of the forest land.?2?According to the model calculation,the microbial retention of the two vegetation types was dominated by ammonia fixation,and the microbial retention capacity of the grassland soil was twice that of the forest land,which again proved the role of soil microbial retention rate in nitrogen mineralization.The microbial retention rate of grassland soil was 0.56 mg·kg-1·d-1,which was mainly dominated by ammonia retention.The soil microbial retention rate of forest land was 0.27 mg·kg-1·d-1,which was still dominated by ammonia.?3?Through experiments,it is concluded that the N2 O emission characteristics of the soils of the two vegetation types in the forest-steppe ecotone.And the N2 O emission rate of grassland soil was 5.885.95 ?g·kg-1·h-1,the N2 O emission rate of forest soil was 5.936.04 ?g·kg-1·h-1.The N2 O emission rate of forest land was higher than that of grassland.The 15N2 O abundance of the two vegetation types in the soil treated with 15NH4 Cl was higher than that of K15NO3.It was presumed that the N2 O emission in the forest grassland interlaced zone mainly came from nitrification.?4?Through nitrogen addition and inhibitor treatment experiments,it was found that DCD can effectively inhibit the nitrification of soil NH4+ and reduce the accumulation of nitrate.The inhibition effect on soil nitrification of grassland decreased with the increase of nitrogen addition.During the 21 d culture period,the nitrification inhibition rates of 10 kg·hm-2,20 kg·hm-2,40 kg·hm-2 urea were 91.66%,79.15% and 58.00%,respectively,and the lower the nitrogen fertilizer concentration,the better the suppression effect of DCD. |
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