ObjectivesThe experiment was to study the effects of long-term straw incorporation on soil ammonium (NH₄⁺) adsorption-desorption characteristics under the surface and subsurface soil level across different cropping systems. Langmuir equation was used to find the maximum adsorption amount (q_max) and adsorption index (b) in order to explore the NH₄⁺ adsorption-desorption difference among different soil types and the relevant impact factors.

MethodsSoil samples of 0–20 cm and 20–40 cm depth were taken from the long-term fertilization experiments in Wangcheng (Hunan Province, double rice rotation), Jinxian (Jiangxi Province, double rice rotation), and Beibei (Chongqing City, rice–wheat rotation) after rice/late rice harvest in October, 2015. The treatments included were CK (no fertilization), NPK (chemical fertilizers only) and NPKS (straw returning with chemical fertilization), and soil ammonium N (NH₄⁺) adsorption-desorption experiment using the sampled soils was conducted in the laboratory. For the NH₄⁺ adsorption experiment, different concentrations of NH₄Cl solution were added in the soil samples. After oscillation and centrifugation, the amount of NH₄⁺ adsorption was the NH₄⁺ content in the filtrate. The above soil samples were then eluted by anhydrous ethanol until no NH₄⁺ could be tested. Adding 0.01 mol/L KCl solution with oscillation and centrifugation, and then we analyzed the NH₄⁺ content in the filtrate as NH₄⁺ desorption amount.

ResultEffects of long-term straw returning on soil N adsorption and desorption characters were different across the three sites. NH₄⁺ adsorption difference was mainly detected in the surface soil. There was no difference of NH₄⁺ adsorption among the treatments and soil layers when NH₄⁺ < 400 mg/L in the balanced solution. However, NH₄⁺ adsorption in the surface layer showed difference among the treatment when NH₄⁺ > 400 mg/L in the balanced solution. At Wangcheng site, N adsorption showed as CK > NPK > NPKS. However, at Beibei, N adsorption showed as CK > NPKS > NPK. N adsorption was found significant higher in the purple soil at Beibei than that in the red soil at Wangcheng or Jinxian sites. NH₄⁺ adsorption was not different among the treatments at Jinxian, where was showed the lowest N adsorption among the three sites. Based on the correlation analysis,q_max was showed significant positive correlation with soil pH and soil cation exchange capacity (CEC), but negative correlation with soil organic matter and total nitrogen. Whereas the correlation ofb value and soil properties was opposite toq_max. For the NH₄⁺ desorption, there was no significant difference among the samples with different treatment from all three sites. NH₄⁺ desorption was higher than the NH₄⁺ adsorption at Wangcheng and Jinxian, whereas maximum NH₄⁺ desorption (541.89–742.38 mg/kg) was much lower than NH₄⁺ adsorption (1003.83–2014.79 mg/kg) at Beibei.

ConclusionsLong-term straw returning shows different impact on soil N adsorption-desorption in different soil types. As the straw returning increase the organic matter content and decrease the adsorption point of a soil, the N adsorption will be improved by long-term straw returning in purple soil on which there are more adsorption spots and higher potassium ion content, but that will be reduced in the red soil where the soil pH is lower and soil organic matter content is higher.