Availability of soil base cations and micronutrients along soil profile after 13-year nitrogen and water addition in a semi-arid grassland

Alteration in the availability of soil base cations and micronutrients is critical to maintain stable ecosystem functioning under the predicted global change scenarios. However, changes in these soil cations and their relationships with soil physiochemical properties along soil profile remain unclear under the combined increasing N deposition and precipitation changes. In this study, the concentrations of soil exchangeable base cations (Ca, Mg, K and Na) and available micronutrients (Fe, Mn, Cu and Zn) were determined along an 80-cm soil profile after 13-year continuous N and water manipulation in a semi-arid grassland. Our results showed that N addition significantly decreased exchangeable Ca (- 25.4%, averaging across the three N addition rates) and Mg (- 7.8%) at the depth of 10 cm while increased available Fe (+ 70.5%), Mn (+ 64.7%), and Cu (+ 26.0%). Besides, the magnitude of the increase or decrease escalated with the rates of additional N. Such pattern was also true for the concentrations of available Fe, Mn and Cu in the 10-20 cm soil layer, but the magnitude of changes was much smaller than in the top 10-cm soil layer. Nevertheless, N addition increased the concentrations of the three available micronutrients across the entire profile, indicating that Fe, Mn and Cu were more sensitive to N addition in subsoils than surface soils. Nitrogen addition significantly reduced soil cation exchange capacity (CEC) in the top 10-cm and soil base saturation (BS) ratio in the top 20-cm soil, while water addition significantly increased soil CEC and BS ratio in the top 20-cm soil. Water addition significantly increased Na (+ 75.1%) in the entire soil profile and increased Ca (+ 14.8%), Mg (+ 12.7%) at the 0-10, 10-20 and 40-60 cm soil layers. Soil pH positively correlated with soil exchangeable Ca, Mg and Na, but negatively with available Fe, Mn and Cu in the upper 20 cm. Soil base cations and CEC positively correlated with clay and silt contents, but negatively with sand content along the profile. These results can extend our understandings on soil cation dynamics to deep soil profile under long-term N and water addition and suggest that precipitation effects should be considered when assessing N deposition effects on these soil cations.