Do enzyme activities during decomposition follow predicted patterns? A test of the conceptual model of litter decay.

Surprisingly, there remains a paucity of research examining specific interactions between the relationship between microbial community behavior and plant litter chemistry during decomposition. A more mechanistic understanding of the relationship between these drivers will ultimately help determine the trajectory of litter decomposition and the conditions in which soils serve as either a source or sink for atmospheric C. In order to examine these relationships, a laboratory incubation was established using _Acer saccharum_ litter and a sandy soil (< 1.5% organic matter). Extracellular enzyme activities ([BETA]-glucosidase, N-acetyl glucosaminidase, leucine-amino peptidase, acid phosphatase, phenol oxidase, and peroxidase) were monitored on a consistent basis along with instantaneous rates of carbon dioxide production, microbial biomass (carbon and nitrogen) and phospholipid fatty acid biomarkers (PLFA), and nitrogen and phosphorus availability. Microbial biomass and microbial respiration peaked within the first week of the experiment. This was likely due to the high availability of water soluble substrates early in decay that can be obtained without the production of extracellular enzymes. [BETA]-glucosidase (BG), N-acetyl glucosaminadase (NAG), and acid phosphatase activities increased quickly following the first week and peaked within the first month (at approximately 15% mass loss). Leucine amino peptidase was not detected during the incubation, which may be due to its strong positive correlation with soil pH, while other hydrolytic enzymes tend to track concentrations of soil organic matter. Phenol oxidase and peroxidase activities were not measurable until the second month of the experiment (> 25% mass loss), likely following the depletion of more labile substrates. A second increase in BG activity was observed between Days 83-111, which may be due to an increase in the availability of cellulose that was previously shielded by lignin, since oxidative enzyme activity was first detected on Day 68. We also observed some shifts in microbial PLFAs along with enzyme activities during decomposition. Prior to the increases in enzyme activity we observed a high proportion of PLFA 18:1[omega]7c, which is a bacterial biomarker. As enzyme activities increased, we observed a decrease in this biomarker and an increase in 18:2[omega]6,9c, a fungal biomarker that was correlated with BG and NAG activity. We did not observe any clear relationships between PLFAs and lignolytic enzyme activity, however. Overall, we observed a distinct functional shift in microbial substrate use that may be associated with either changes in composition of the microbial community or community shifts in enzyme production

Dynamic relationships between microbial biomass, respiration, inorganic nutrients and enzyme activities: informing enzyme based decomposition models

We re-examined data from a recent litter decay study to determine if additional insights could be gained to inform decomposition modeling. Rinkes et al. (2013) conducted 14-day laboratory incubations of sugar maple (Acer saccharum) or white oak (Quercus alba) leaves, mixed with sand (0.4% organic C content) or loam (4.1% organic C). They measured microbial biomass C, carbon dioxide efflux, soil ammonium, nitrate, and phosphate concentrations, and β-glucosidase (BG), β-N-acetyl-glucosaminidase (NAG), and acid phosphatase (AP) activities on days 1, 3, and 14. Analyses of relationships among variables yielded different insights than original analyses of individual variables. For example, although respiration rates per g soil were higher for loam than sand, rates per g soil C were actually higher for sand than loam, and rates per g microbial C showed little difference between treatments. Microbial biomass C peaked on day 3 when biomass-specific activities of enzymes were lowest, suggesting uptake of litter C without extracellular hydrolysis. This result refuted a common model assumption that all enzyme production is constitutive and thus proportional to biomass, and/or indicated that part of litter decay is independent of enzyme activity. The length and angle of vectors defined by ratios of enzyme activities (BG/NAG versus BG/AP) represent relative microbial investments in C (length), and N and P (angle) acquiring enzymes. Shorter lengths on day 3 suggested low C limitation, whereas greater lengths on day 14 suggested an increase in C limitation with decay. The soils and litter in this study generally had stronger P limitation (angles > 45˚). Reductions in vector angles to < 45˚ for sand by day 14 suggested a shift to N limitation. These relational variables inform enzyme-based models, and are usually much less ambiguous when obtained from a single study in which measurements were made on the same samples than when extrapolated from separate studies

Field and lab conditions alter microbial enzyme and biomass dynamics driving decomposition of the same leaf litter

Fluctuations in climate and edaphic factors influence field decomposition rates and preclude a complete understanding of how microbial communities respond to plant litter quality. In contrast, laboratory microcosms isolate the intrinsic effects of litter chemistry and microbial community from extrinsic effects of environmental variation. Used together, these paired approaches provide mechanistic insights to decomposition processes. In order to elucidate the microbial mechanisms underlying how environmental conditions alter the trajectory of decay, we characterized microbial biomass, respiration, enzyme activities, and nutrient dynamics during early (< 10% mass loss), mid- (10-40% mass loss), and late (> 40% mass loss) decay in parallel field and laboratory litter bag incubations for deciduous tree litters with varying recalcitrance (dogwood < maple < maple-oak mixture < oak). In the field, mass loss was minimal (< 10%) over the first 50 days (January-February), even for labile litter types, despite above-freezing soil temperatures and adequate moisture during these winter months. In contrast, microcosms displayed high C mineralization rates in the first week. During mid-decay, the labile dogwood and maple litters in the field had higher mass loss per unit enzyme activity than the lab, possibly due to leaching of soluble compounds. Microbial biomass to litter mass (B:C) ratios peaked in the field during late decay, but B:C ratios declined between mid- and late decay in the lab. Thus, microbial biomass did not have a consistent relationship with litter quality between studies. Higher oxidative enzyme activities in oak litters in the field, and higher nitrogen (N) accumulation in the lab microcosms occurred in late decay. We speculate that elevated N suppressed fungal activity and/or biomass in microcosms. Our results suggest that differences in microbial biomass and enzyme dynamics alter the decay trajectory of the same leaf litter under field and lab conditions

Nitrogen alters microbial enzyme dynamics but not lignin chemistry during maize decomposition

International audienceIncreases in nitrogen (N) availability reduce decay rates of highly lignified plant litter. Although microbial responses to N addition are well documented, the chemical mechanisms that may give rise to this inhibitory effect remain unclear. Here, we ask: Why does increased N availability inhibit lignin decomposition? We hypothesized that either (1) decomposers degrade lignin to obtain N and stop producing lignin-degrading enzymes if mineral N is available, or (2) chemical reactions between lignin and mineral N decrease the quality of lignin and limit the ability of decomposers to break it down. In order to test these hypotheses, we tracked changes in carbon (C) mineralization, microbial biomass and enzyme activities, litter chemistry, and lignin monomer concentrations over a 478-day laboratory incubation of three maize genotypes differing in lignin quality and quantity (F(292)bm(3) (high lignin) < F-2 (medium lignin) < F(2)bm(1) (low lignin)). Maize stem internodes of each genotype were mixed with either an acidic or neutral pH sandy soil, both with and without added N. Nitrogen addition reduced C mineralization, microbial biomass, and lignin-degrading enzyme activities across most treatments. These dynamics may be due to suppressed fungal growth and reduced microbial acquisition of lignin-shielded proteins in soils receiving N. However, N addition alone did not significantly alter the quantity or quality of lignin monomers in any treatment. Our results suggest that abiotic interactions between N and phenolic compounds did not influence lignin chemistry, but mineral N does alter microbial enzyme and biomass dynamics, with potential longer-term effects on soil C dynamics