Dynamics of storage and relative availability of soil inorganic nitrogen along revegetation chronosequence in the loess hilly region of China

Abstract

In the Loess Hilly Region of China, the widespread conversion of farmland to forestland and grassland has resulted in great changes in soil carbon (C) and nitrogen (N) storage in the shallow soil layers. However, knowledge regarding changes in the status and relative availability of inorganic nitrogen (IN) is still limited. To determine the responses of soil IN storage and its relative availability to revegetation patterns and chronosequence, a 0–200 cm soil profile was collected from two typical vegetation restoration patterns in the heartland of the region, artificial forestland (Robinia pseudoacacia) and natural grassland, with a stand age sequences of 15, 20, 25, 30 and 40 years. The farmland near the sites was selected as a control. As a whole, the NH4-N, NO3-N and total inorganic nitrogen (TIN) concentrations and stocks in the forestlands were significantly higher than those in the grasslands, especially in the topsoil (0–10 cm). With increasing revegetation chronosequence, afforestation improved soil NH4–N, NO3–N and TIN concentrations and stocks until Robinia pseudoacacia reached maturity at an age of 25–30, but abandoned on former farmland had minor effects on soil IN. After revegetation, soil NH4–N was higher than NO3–N, and the NH4–N/NO3–N ratio (ANR) was greater than 1 and increased with soil depth. The soil at 40–200 cm accounted for 60–90% of the total IN stock of the 0–200 cm soil profile, and the soil at 100–200 cm accounted for more than 35% of the total levels. Moreover, soil physicochemical properties were significantly correlated with soil IN variables, and ANR had a greater correlation coefficient with soil physicochemical properties than the other IN variables. It was concluded that afforestation was a better choice for soil IN restoration of degraded land, and subdeep and deep soil provided an extremely important resource for evaluating soil IN pools and their relative availability. Furthermore, soil IN dynamics, particularly ANR, could be regarded as indicators for assessing the dynamics of soil fertility.