Trade-off between soil moisture and species diversity in semi-arid steppes in the Loess Plateau of China.

Effectively balancing soil moisture and biodiversity restoration remains a contentious issue for managers and researchers in the Loess Plateau region of China, even after many years of restoration efforts. We conducted a regional study on the trade-off between soil moisture and species diversity using spatial grid sampling in a semi-arid steppe (200-300 mm annual precipitation) in the northwest Loess Plateau. Results reveal that only soil moisture between 20 and 60 cm depth was significantly correlated with diversity indexes. Root-mean-square deviation (RSMD, the index of the soil water-biodiversity relationship) increased by monotonous linear trends with soil moisture in 20-60 cm depth. The linear relationship for Shannon Wiener diversity index (SD) was stronger than for species richness index (SR). When soil moisture in 20-60 cm depth was lower than 6-8%, RSMD often was less than zero, representing the trade-off relationship. However, synergism was more common as the soil moisture increased beyond 6-8%. The overall trends and the soil moisture threshold (6-8%) did not differ significantly between sites with different vegetation cover and aspect, though there were differences in the relative ratio of trade-off and synergism samples. Comparing results from sampling at different scales in the Loess Plateau suggests 6-8% soil moisture in 200-300 mm precipitation gradient, consistent with 370 mm rainfall depth in 250-550 mm precipitation gradient, might be a scale-independent threshold driving the soil moisture-biodiversity relationship from trade-off to synergism in the region.