Leaching of soil legacy nitrogen in intact soil columns and significance of soil macropore structure

Excessive application of chemical fertilizers accumulates nitrogen in soil (soil legacy nitrogen), and its release has a long-term impact on environmental quality. The information regarding leaching of soil legacy nitrogen and role of soil pore structure is scarce. Fifteen undisturbed soil cores with a depth of ~200 mm were collected from five paddy fields in a subtropical area of China, and soil pore structure was characterized with X-ray computed tomography. The batch leaching column experiments were conducted to investigate the nitrogen leaching from the soil cores. In the leachate, inorganic nitrogen (NO₃^--N and NH₄⁺-N) accounted for 73-85 % of total dissolved nitrogen (TDN), which showed small variations among the five sampling sites. The NO₃^--N, NH₄⁺-N and TDN concentrations in the leachate over the number of leaching times could be well fitted with the Gaussian, exponential or linear models, suggesting that different nitrogen forms showed variable leaching dynamics. NO₃^--N and TDN leaching losses continued to increase with the number of leaching times, whereas NH₄⁺-N leaching from the soil had a threshold value. Combination of changes in soil nitrogen content after leaching, the lagging effect of soil legacy nitrogen may be partly attributed to the continuous transformation of NH₄⁺-N and organic nitrogen into NO₃^--N in the soils. Besides the nitrogen content in the soil, the most important factor of soil pore structure controlling NO₃^--N and TDN leaching loss was the fractal dimension; NH₄⁺-N was mainly controlled by connectivity. The present study highlighted the importance of soil pore structure and nitrogen format transformation in the leaching of soil legacy nitrogen.