Uncertain future soil carbon dynamics under global change predicted by models constrained by total carbon measurements.

Pool-based carbon (C) models are widely applied to predict soil C dynamics under global change and infer underlying mechanisms. However, it is unclear about the credibility of model-predicted C pool size, decay rate (k), and/or microbial C use efficiency (e) as only data on bulked total C is usually available for model constraining. Using observing system simulation experiments (OSSE), we constrained a two-pool model using simulated data sets of total soil C dynamics under topical hypotheses on responses of soil C dynamics to warming and elevated CO2 (i.e., global change scenarios). The results indicated that the model predicted great uncertainties in C pool size, k, and e under all global change scenarios, resulting in the difficulty to correctly infer the presupposed "real" values of those parameters that are used to generate the simulated total soil C for constraining the model. Furthermore, the model using the constrained parameters generated divergent future soil C dynamics. Compared with the predictions using the presupposed real parameters (i.e., the real future C dynamics), the percentage uncertainty in 100-yr predictions using the constrained parameters was up to 45% depending on global change scenarios and data availability for model-constraining. Such great uncertainty was mainly due to the high collinearity among the model parameters. Using pool-based models, we argue that soil C pool size, k, and/or e and their responses to global change have to be estimated explicitly and empirically, rather than through model-fitting, in order to accurately predict C dynamics and infer underlying mechanisms. The OSSE approach provides a powerful way to identify data requirement for the new generation of model development and test model performance.

Relating microbial community structure to functioning in forest soil organic carbon transformation and turnover.

Forest soils store vast amounts of terrestrial carbon, but we are still limited in mechanistic understanding on how soil organic carbon (SOC) stabilization or turnover is controlled by biotic and abiotic factors in forest ecosystems. We used phospholipid fatty acids (PLFAs) as biomarker to study soil microbial community structure and measured activities of five extracellular enzymes involved in the degradation of cellulose (i.e., ß-1,4-glucosidase and cellobiohydrolase), chitin (i.e., ß-1,4-N-acetylglucosaminidase), and lignin (i.e., phenol oxidase and peroxidase) as indicators of soil microbial functioning in carbon transformation or turnover across varying biotic and abiotic conditions in a typical temperate forest ecosystem in central China. Redundancy analysis (RDA) was performed to determine the interrelationship between individual PFLAs and biotic and abiotic site factors as well as the linkage between soil microbial structure and function. Path analysis was further conducted to examine the controls of site factors on soil microbial community structure and the regulatory pathway of changes in SOC relating to microbial community structure and function. We found that soil microbial community structure is strongly influenced by water, temperature, SOC, fine root mass, clay content, and C/N ratio in soils and that the relative abundance of Gram-negative bacteria, saprophytic fungi, and actinomycetes explained most of the variations in the specific activities of soil enzymes involved in SOC transformation or turnover. The abundance of soil bacterial communities is strongly linked with the extracellular enzymes involved in carbon transformation, whereas the abundance of saprophytic fungi is associated with activities of extracellular enzymes driving carbon oxidation. Findings in this study demonstrate the complex interactions and linkage among plant traits, microenvironment, and soil physiochemical properties in affecting SOC via microbial regulations.

Irrigation and enhanced soil carbon input effects on below-ground carbon cycling in semiarid temperate grasslands.

Global climate change is generally expected to increase net primary production, resulting in increased soil carbon (C) inputs. To gain an understanding of how such increased soil C inputs would affect C cycling in the vast grasslands of northern China, we conducted a field experiment in which the responses of plant and microbial biomass and respiration were studied. Our experiment included the below-ground addition of particulate organic matter (POM) at rates equivalent to 0, 60, 120 and 240 g C m(-2), under either natural precipitation or under enhanced precipitation during the summer period (as predicted for that region in recent simulations using general circulation models). We observed that addition of POM had a large effect on soil microbial biomass and activity and that a major part of the added C was rapidly lost from the system. This suggests that microbial activity in the vast temperate grassland ecosystems of northern China is energy-limited. Moreover, POM addition (and the associated nutrient release) affected plant growth much more than the additional water input. Although we performed no direct fertilization experiments, the response of plant productivity to POM addition (and associated release of nutrients) leads us to believe that plant productivity in the semiarid grassland ecosystems of northern China is primarily limited by nutrients and not by water.

Magnesium nutrition and photosynthesis in Pinus radiata: clonal variation and influence of potassium.

Magnesium (Mg) nutrition and photosynthesis were studied in clones of Pinus radiata D. Don grown in sand culture for 21 weeks at four Mg concentrations (0.008, 0.04, 0.2 and 0.4 mM) and three potassium (K) concentrations (0.25, 0.5 and 2.5 mM). We found significant clonal variation in Mg nutrition of P. radiata. Plants grown at 0.04 mM [Mg] or less showed pronounced visible symptoms of foliar Mg deficiency. Net photosynthetic rate and leaf conductance were closely related to shoot Mg concentrations below a concentration of 0.6 mg Mg g(DW) (-1). Potassium enhanced the development of visible symptoms of foliar Mg deficiency. At the lowest Mg concentration tested (0.008 mM), the severity of needle chlorosis and necrosis increased with increasing K concentration in the culture solution. With increasing Mg concentration, 2.5 mM [K] in the culture solution markedly increased root Mg concentration, but decreased shoot Mg concentration, suggesting that excessive K inhibited Mg mobilization from roots to shoots. Rates of growth and photosynthesis were both severely inhibited at 0.008 mM [Mg].