Belowground net primary productivity and biomass allocation in response to different restoration measures in a salt-alkali-degraded Songnen meadow

Abstract

The understanding of belowground productivity restoration and biomass allocation is especially important for the restoration of salt-alkali-degraded meadows. Despite this, it remains unclear how belowground net primary productivity (BNPP) and biomass allocation respond to combined restoration measures. To assess the effects of different combined restoration measures on BNPP and biomass allocation, a field experiment was carried out in a salt-alkali-degraded meadow from 2012 to 2014. The restoration treatments included: free grazing (FG), enclosure with no-ploughing (E), enclosure with ploughing alone (EP), enclosure with ploughing and N addition (EPF), enclosure with ploughing and mulching (EPM), and enclosure with ploughing plus both N addition and mulching (EPFM). Our results across all three years showed that relative to FG, all the other restoration measures, whether single or combined, significantly increased aboveground net primary productivity (ANPP), BNPP and net primary productivity (NPP). The response of BNPP to different restoration measures was stronger than the ANPP response, resulting in a higher root fraction. The trends were clearer in the EPF, EPM and EPFM treatments, suggesting that more C was allocated to belowground roots with N addition and mulching. The strong increases in NPP and BNPP and belowground biomass allocation in the EPF, EPM and EPFM treatments were mainly due to significant increases in the density of important species (e.g. Chloris virgata and Puccinellia tenuiflora) and soil moisture, and strong decreases in soil pH and EC. Our findings demonstrate that vegetation cover, species richness and productivity (ANPP + BNPP) should be considered as important indicators when evaluating the recovery of severely degraded grassland. Our results further suggest that the combined measures, including enclosure and ploughing plus N addition and mulching, are effective ways to accelerate restoration in salt-alkali-degraded meadows.