Organic Nutrients Induced Coupled C- and P-Cycling Enzyme Activities During Microbial Growth in Forest Soils

Besides environmental and soil physical drivers, the functional properties of microbial populations, i. e., growth rate, enzyme production, and maintenance requirements are dependent on the microbes' environment. The soil nutrition status and the quantity and quality of the substrate input, both infer different growth strategies of microorganisms. It is uncertain, how enzyme systems respond during the different phases of microbial growth and retardation in soil. The objective of this study was to uncover the changes of microbial functioning and their related enzyme systems in nutrient-poor and nutrient-rich beech forest soil during the phases of microbial growth. We determined microbial growth via kinetic approach by substrate-induced respiratory response of microorganisms, enabling the estimation of total, and growing biomass of the microbial community. To induce microbial growth we used glucose, while yeast extract simulated additional input of nutrients and factors indicating microbial residues (i.e., necromass compounds). Microbial growth on glucose showed a 12–18 h delay in associated enzyme activity increase or the absence of distinct activity responses (Vmax). β-glucosidase and chitinase (NAG) demonstrated clear differences of Vmax in time and between P-rich and P-poor soils. However, during microbial growth on glucose + yeast extract, the exponential increase in enzymatic activity was clearly stimulated accompanied by a delay of 8–12 h, smoothing the differences in nutrient-acquisition dynamics between the two soils. Furthermore, cross-correlation of β-glucosidase and acid phosphatase between the two sites demonstrated harmonized time constraints, which reflected the establishment of comparable and balanced enzymatic systems within the decomposition network

Leaching of Phosphomonoesterase Activities in Beech Forest Soils: Consequences for Phosphorus Forms and Mobility

Phosphomonoesterases play an important role in the soil phosphorus (P) cycle since they hydrolyze P monoester to phosphate. Their activity is generally measured in soil extracts, and thus, it remains uncertain how mobile these enzymes are and to which extent they can be translocated within the soil profile. The presence of phosphomonoesterases in soil solutions potentially affects the share of labile dissolved organic P (DOP), which in turn would affect P leaching. Our study aimed at assessing the production and leaching of phosphomonoesterases from organic layers and topsoil horizons in forest soils and its potential effect on dissolved P forms in leachates obtained from zero-tension lysimeters. We measured phosphomonoesterase activities in leached soil solutions and compared it with those in water extracts from litter, Oe/Oa, and A horizons of two beech forests with a contrasting nitrogen (N) and P availability, subjected to experimental N x P fertilization. In addition, we determined phosphate and DOP. In soil solutions leached from litter, Oe/Oa, and A horizons, phosphomonoesterase activities ranged from 2 to 8 mu mol L-1 h(-1) during summer, but remained below detection limits in winter. The summer values represent 0.1-1% of the phosphomonoesterase activity in soil extracts, indicating that enzymes can be translocated from organic layers and topsoils to greater soil depths. Activities of phosphomonoesterases obtained by water extracts were greater in the organic layer of the P-poor site, while activities of those in soil solutions were similar at the two sites. Nitrogen addition increased phosphomonoesterase activities in leached soil solutions of the organic layer of the N- and P-poor soil. Using a modeling approach, we estimated that approx. 76% of the initial labile DOP was hydrolyzed to dissolved inorganic P within the first 24 h. Back calculations from measured labile DOP revealed an underestimation of approx. 15% of total dissolved P, or 0.03 mg L-1. The observed leaching of phosphomonoesterases implies that labile organic P could be hydrolyzed in deeper soil horizons and that extended sample storage leads to an underestimation of the contribution of DOP to total dissolved P leaching. This has been neglected in the few field studies measuring DOP leaching

Manufacturing triple-isotopically labeled microbial necromass to track C, N and P cycles in terrestrial ecosystems

The functional relevance of microbial necromass in terrestrial biogeochemical cycles remains one of the unresolved mysteries of element cycling in ecosystems, especially considering the high microbial abundance and turnover in soil. We therefore established a protocol to manufacture multi-isotope (14C, 15N and 33P) labeled microbial necromass to comprehensively track the turnover of microbial necromass elements within element cycles. This protocol encompasses the i) microbial cultivation of Pseudomonas kilonensis ACN4 (Gram-negative) and Bacillus licheniformis DSM13 (Gram-positive) on labeled minimal medium as well as fungal cultivation of Hypsizygus tessulatus on a complex yeast medium, ii) quantification of radio- (14C, 33P) and stable (15N) isotope incorporation as well their cellular pool partitioning, and iii) determination of element and tracer isotope uptake efficiency. We achieved 1 g of bacterial biomass per liter minimum medium within 24 h and 2.9 g l-1 fungal biomass in complex medium within 18 d. This production rate enabled us to produce more than 100 g of necromass within only one half-life time of 33P, including post-harvest processing. Isotope uptake and incorporation for 33P ranged from 10 to 73%, for 15N from 24 to 52%, and for 14C from 12 to 23%. Each of the cultivated species showed individual patterns of tracer element uptake. The nutritional value of the carbon- (C), nitrogen- (N) and phosphorus- (P) labeled microbial necromass was characterized by a water-based, necromass speciesspecific partitioning scheme with subsequent elemental analysis of the pools. We separated Gram-negative, Gram-positive and fungi’s cellular pools to characterize element and tracer partitioning among dissolved versus particulate fractions. That is essential because these properties subsequently affect the respective pool's availability for ecosystem nutrition. Our procedure allows a defined production of microorganism-based necromass, enabling versatile use to determine necromass-related nutrient fluxes in terrestrial ecosystem studies