Effects Of Different Tillage Applications On Nitrogen Transformations Of Wheat Residues In Northern Dryland Of China

Conservation tillage?NT?,a no-tillage approach where only crop residues retention,has been proposed as a way to enhance soil quality,reduce soil erosion and incorporate nutrients that have been widely adopted in Northern China.Crop straw return is an effective and sustainable management practice for tackling and promoting soil quality and improving crop yield.Changes in soil nitrogen are influenced by different agronomy practices,such as tillage,fertilizer application and straw return.Therefore,the objective of the current study is mainly focus on the ¹⁵N-labelled wheat nitrogen transformation from wheat residues to soil and the recovery of residue nitrogen under the no-tillage practices,which could provide theoretical basis for the determination of reasonable wheat residue nitrogen.A long-term no-tillage and conventional tillage experiment was counduct to follow the cycling of N derived from wheat residues in a dry-land winter wheat in northern China.?1?Effects of the different tillage on the uptake and the utilization dynamics of residues N by winter wheat.The concentration of residue ¹⁵N in different organs of wheat was highest in grain and lowest in leaf.Compared with CT treatment,NT treatment remarkably increased the uptake of straw N by roots and leaves,which increased by 36.5%and 31.2%,respectively.The N uptake of wheat comes from residue ¹⁵N,which accounts for about 1.5%of total nitrogen absorption under the two treatments.The straw N recovery efficiency by NT and CT treatments were 6.26%and 6.25%,respectively.The results indicated that the soil nitrogen and fertilizer nitrogen are the main nitrogen sources of wheat than straw.?2?The loss of straw N through N₂O and NH₃ under long-term conservation tillageIn the whole growth period of wheat,the N₂O and NH₃ emission fluxes were higher under NT treatment than CT treatment.The total N₂O fluxs in NT(0.487 kg N ha^-1)was 11%higher than CT(0.433 kg N ha^-1).The NH₃ volatilization rate was significantly influenced by different tillage,especially in wheat reviving stage.?3?Dynamic changes of residue ¹⁵N in soil labile nitrogen pools in different tillage soilsSoil samples were collected in 0-20cm soil layer from long-term NT and CT treatments.There were significant differences in the dynamics of nitrogen pools in different tillage soils.Compared to conventional tillage,conservation tillage showed a higher ¹⁵N concentration in soil total nitrogen.The ratio of mineral ¹⁵N to mineral N in the soil was presented higher in NT+S treatment than CT+S treatment.A greater proportion of straw ¹⁵N was recovered in the PON fraction under two treatments.After 56d,13.9%of the straw ¹⁵N was recovered in PON,6.9%was recovered in MBN and 3.1%was recovered in WEON under NT+S treatment.Meanwhile,12.7%of the straw ¹⁵N was recovered in PON,6.7%was recovered in WEON and 4.3%was recovered in MBN under CT+S treatment.The results indicated that PON is the main pathy to provide mineralized N in agricultural soil,and long-term no-tillage with straw retention significantly improves the transformation of straw N to PON.?4?The spatial distribution of straw N in soil under long-term conservation tillageThe distribution of residue ¹⁵N in 0-100cm soil layer under NT and CT treatments showed the same trend and decreased with soil depth.The residue ¹⁵N mainly exists in 0-60cm soil layer,which account for more than 90%of the total residual residue ¹⁵N in the 0-100 cm soil layer in the NT and CT treatment.The total residue ¹⁵N under NT and CT treatments was 11.94 kg ha^-1 and 13.26 kg ha^-1,respectively,accounting for 46.9%and 53.6%of the total application residue ¹⁵N,respectively.After a growing season,6.26%of the residue ¹⁵N was absorbed by the in-season wheat,47%remained in the soil,and 47%existed in gaseous and undecomposed form under NT treatment.The 6.25%of residue¹⁵N was absorbed by in-season wheat,54%exist in soil,and 40%existed in gas and undecomposed form.