Soil Nitrogen Mineralization In Alfalfa Pasture

Alfalfa,the biological nitrogen provider,is one of the most important high quality legume forages in pastoral agriculatural system.Soil nitrogen mineralization is always the heated topic in agricultural and ecological fields.The understanding of the nitrogen mineralization in alfalfa pasture is therefore critical for inherent soil N fertility,crop sequence selection and N fertilization requirements.Two experiments were designed to estimate the potential N mineralization including a chemical extraction experiment and a laboratory incubation experiment.Chemical experiment was used to estimate the difference of the capacity of three different extractions(hot water,KCl and CaCl₂). Laboratory experiment was designed to challenge the idea that the initial mineral nitrogen, nitrogen fertilizer added and the plant coverage may control nitrogen mineralization.Soils were collected from 5 and 9-year alfalfa stands at regreening and first cutting period in Qingyang Loess Plateau Station of Lanzhou University.The results were as follows:1.There was significant correlation(P<0.01)of the amount of NO₃^--N extracted by hot water,1 mol/L KCl and 0.01 mol/L CaCl₂.Similar situations were observed with NH₄⁺-N and soil mineral nitrogen.Significant correlation coefficients(P<0.05)were found between mineral N and N uptake by plants.Hot water extraction method gave the highest correlation with N uptake.The results suggested that hot water,1 mol/L KCl and 0.01 mol/L CaCl₂ were efficient extractions which can be used to extract soil mineral N,and mineral nitrogen contents extracted by three extractions should reflect the level of mineral nitrogen and ability of soil to supply available N.0.01 mol/L CaC1₂ extracted more NO₃^--N than hot water and KCl,and hot water can extract NH₄⁺-N efficiently.In contrast,1 mol/L KCl extracted the least amount of NO₃^--N and NH₄⁺-N.2.Initial leaching of inorganic nitrogen and addition of N fertilizer significantly affected N mineralization rate of soil from 5-year lucerne stands,but only slightly affected the soil from 9-year lucerne stands.Planted ryegrass significantly increased net N mineralization rate both in soils from 5 and 9-year lucerne stands.3.Net N mineralization rate(0.16 mg/kg/day)and net N nitrification(10.76 mg/kg)of 9-year lucerne stands was obviously higher than those of 5-year Lucerne stands which are 0.02 mg/kg/day and 0.59 mg/kg.This suggests that N supplying capacity could increase with the increasing age of alfalfa stand.Furthermore,N fertilizer addition could enhance the nitrification of soil.4.There were no significant correlations(P＞0.05)between total soil nitrogen,net N mineralization and N uptake by ryegrass.In contrast,there was significant correlation (P＜0.05)between soil microbial biomass nitrogen and net N mineralization.Thus,soil microbial biomass serves as a sink and source of N and plays an important role in regulating soil available N.