Microbial nitrogen and phosphorus mineralization and microbial biomass stoichiometry as dependent on ratios of carbon, nitrogen and phosphorus in soils of temperate forests.
Christine Heuck (04/2014-08/2018)
Betreuer: Marie Spohn
This thesis focuses on the question of how different ratios of carbon (C), nitrogen (N) and phosphorus (P) in soils of temperate forests influence soil microbial C:N:P ratios as well as their net N and P mineralization. This addresses two gaps in the knowledge of the meaning of C:N:P stoichiometry, i.e. the relationship of C, N and P, in soil. Although microbial biomass C:N:P stoichiometry is thought to be globally constrained, it is unknown whether these constraints apply to fertilized soils. Moreover, the research on the relationship of organic layer C:N:P stoichiometry to microbial net N and P mineralization is fragmentary. The influence of organic layer C:N or C:P ratios on net N and P mineralization has most frequently been examined in fresh litter, thus ignoring Oe and Oa horizons of organic layers as well as soil N:P ratios. In the course of studying net mineralization, special attention was paid to net P mineralization. This is related to the current discussion on the impact of continuously increased atmospheric N depositions on the P demand of temperate forests, which sparked great interest in the details of P cycling in temperate forest soils. Twelve temperate forests in Europe and the Eastern USA were sampled for this thesis. The inclusion of deciduous and coniferous forests with different soil N and P contents ensured variability in soil C:N:P stoichiometry. The constraints of microbial C:N:P stoichiometry were tested with respect to short-term and long-term changes of soil nutrient availability. In a short-term laboratory incubation experiment with full-factorial design, microbial C, N and P were determined after addition of easily available C, N and P to two exemplary soils (nutrient rich vs. poor). Moreover, microbial C, N and P was determined in long-term N fertilization experiments (> 25 years). In both cases, the chloroform-fumigation extraction method was used to measure microbial C, N and P. Net N and P mineralization were determined in different horizons (Oi, Oe, Oa, Oe+Oa) of the organic layers of all studied forests. Net N and P mineralization were derived from the increase of N and P concentrations over time in regularly prepared soil extracts during laboratory incubations (3 months). Net P mineralization was examined in more detail in two experiments. The microbial mineralization of a both C- and P-rich compound was analyzed in a short-term laboratory incubation of soil with either 14C or 33P labeled glucose-6-phosphate. Moreover, phosphatase activity as determined with a fluorogenic substrate was related to net P mineralization in the long-term N fertilization experiments. The microbial biomass C:N:P stoichiometry of soils exposed to different element inputs was mostly constrained. There was only one case of increased variability of microbial C:N:P stoichiometry in a nutrient-poor B horizon treated with short-term C, N and P amendments. The relationship between soil C:N:P ratios and microbial net N and P mineralization was strong and depended on the decomposition state of organic matter. Net N mineralization occurred below certain threshold soil C:N ratios (28 40); in some cases there were also threshold soil N:P ratios (42 60). Below the thresholds, net N mineralization increased with decreasing soil C:N and, in a few instances, N:P ratios. Net P mineralization had both threshold soil C:P (1000 1400) and N:P ratios (40 44) below which it increased with decreasing C:P and N:P ratios. Oi horizons had higher threshold ratios and stronger increases of net N and P mineralization with decreasing C:N, C:P or N:P ratios than Oa horizons. No clear trends were found for the intermediate Oe horizons and long-term N fertilization of forests partly obfuscated stoichiometric relationships. Regarding net P mineralization, soil microorganisms recovered more C than P from glucose-6-phosphate (14C recovery: 28 37%, 33P recovery: 1 6%) thus P was released into the soil. Phosphatase activity increased (on average +260%) in N-fertilized organic layers, but this did not coincide with increased net P mineralization except in one case. In conclusion, the constraints of microbial biomass C:N:P stoichiometry were robust to variance in soil C:N:P stoichiometry, whereas microbial net N and P mineralization in soils of temperate forests was largely determined by it. In the future, the occurrence and amount of net N or P mineralization may be assessed by simply determining C:N:P ratios of soils. However, reliable estimates will depend on a larger database than presented here. Especially, more research of coniferous and temperate forests is needed. Moreover, net P mineralization driven by the microbial C demand in temperate forest soils may be a common phenomenon that is beneficial for plant nutrition. The increase of phosphatase activity seems to indicate increased P demand in N-fertilized forests. Under this condition, additionally mineralized P due to the increased phosphatase activity is more likely to be consumed quickly by plants or microorganisms than to accumulate in soil.