Applied monosodium glutamate wastewater promoting poplar growth, improving soil active organic carbon and carbon pool management index

Abstract: Soil total organic carbon (TOC) is a stable and long-lasting carbon source and contains nearly all nutrients for plant growth, and therefore is a meaningful index for evaluating soil fertility. The quantity and quality of soil organic carbon influence soil potential productivity and reflect the spatial distribution and succession of plant community. However, it is not easy to detect short and medium term changes of total TOC due to the variation of soil background and natural level, and thus techniques that measure meaningful fractions of TOC should be used. The active organic carbon (AOC) as the component of TOC is a more sensitive indicator for environmental change. Although the proportion of AOC to TOC is very small, the AOC is closely related to soil productivity and plays important role in soil nutrient supply. Furthermore, the carbon pool management index (CPMI) calculated from changes in labile and total carbon relative to a reference soil shows to be a useful indicator for describing soil fertility. Therefore, analyzing the change rule of AOC and CPMI has important guiding significance for fertilization. In order to explore the biological effect of monosodium glutamate wastewater on soil improvement and determine the appropriate application level of inorganic fertilizer and monosodium glutamate wastewater for improving poplar growth, a field experiment including 5 treatments, i.e. CK (neither urea nor monosodium glutamate wastewater was applied), N100 (100% of nitrogen was provided by urea), M10N90 (10% and 90% of nitrogen were provided by monosodium glutamate wastewater and urea, respectively), M30N70 (30% and 70% of nitrogen were provided by monosodium glutamate wastewater and urea, respectively) and M50N50 (50% and 50% of nitrogen were provided by monosodium glutamate wastewater and urea, respectively) was performed. The present study was conducted to determine the effects of different treatments on TOC, AOC component and effective rate, CPMI in soil and poplar growth. Results indicated that in comparison to N100 treatment, the TOC content significantly increased under the treatments of M10N90, M30N70 and M50N50, and M50N50 treatment was apparently higher than other treatments. The AOC, dissolved organic carbon (DOC) and microbial biomass carbon (MBC) contents and their effective rates in M30N70 treatment all reached the maximum, of which AOC content was 43.58%, 59.35%, 20.71% and 9.65% higher than CK, N100, M10N90 and M50N50 respectively and DOC content was 62.71%, 52.38%, 28.00% and 14.29% higher than CK, N100, M10N90 and M50N50 respectively. In the meantime, the CPMI was 192.56 under the M30N70 treatment, which was 89.45, 43.58 and 30.86 higher than the treatments of N100, M10N90 and M50N50, respectively. Additionally, the average growth rates of diameter at breast height (DBH), tree height and volume in M30N70 treatment were 23.40%, 21.62% and 45.05%, respectively, which were also obviously higher than other treatments. However, in M10N90 and M50N50 treatments, less effects on AOC component and CPMI in the forest soil and tree growth rates were observed compared with M30N70 treatment. The correlation analysis showed that volume growth rate was extremely significantly correlated with AOC, AOC effective rate, DOC, MBC and CPMI (P<0.01). These results suggest that monosodium glutamate wastewater combined with inorganic fertilizer, especially M30N70 treatment, is beneficial to increase AOC content and CPMI of woodland soil, improve soil quality and fertility, and promote the growth of poplar. The results can provide theoretical foundation for the effective utilization of monosodium glutamate wastewater in forestry. In addition, we can speculate the monosodium glutamate wastewater combined with inorganic fertilizer would have broad application prospects in forest cultivation.