Changes of microbial turnover and plant uptake of soil amino acids during conversion of a natural forest to a spruce plantation on the eastern Tibetan plateau, China

It is well recognized that amino acids play an important role in soil nitrogen (N) cycling and contribute to plant N supply in forests. Land-use change is likely to alter soil amino- acid cycling and the role of amino acids in plant nutrition by influencing multiple abiotic and biotic factors, which, however, remains poorly understood. Here, by using ¹⁵N labeling methods, we quantified effects of land-use change from a natural forest to a spruce plantation on the uptake and turnover of amino acids by soil microbes, as well as trees' ability to take up amino acids on eastern Tibetan, China. Forest conversion from the natural forest to the spruce plantation significantly decreased soil inorganic N availability and N transformation rates, while increased the ratio of amino acids to inorganic N concentration. A soil incubation experiment with ¹⁵N-labeled-glycine addition in the laboratory showed that forest conversion inhibited gross consumption of amino acids and facilitated recycling of amino acids within microbial biomass. Furthermore, an in situ ¹⁵N-labeling experiment revealed that forest conversion significantly increased the contribution of soil amino acids to the total N uptake by plants, which was attributed to the increased proportion of amino acids in soil available N pools. Collectively, our results demonstrated that forest conversion from natural forests to plantations impacted turnover of amino acids by soil microbes, which consequently influenced plant uptake of amino acids. Our study further highlights the important role of amino acids as a N source for subalpine coniferous plantations and has implications for formulating management practices to maintain long-term productivity and stability of plantations.