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Concern over climate ciiange has stimuiated interest in the temperature and moisture dependence of sou organic matter decomposition. In particuiar, there has been intense debate in relation to the factors that determine the temperature dependence of C mineralization. We examined temperature and moisture responses of C and N mineralization in an 85-d laboratory incubation (factorial combination of four temperatures [5, 12, 18, 25°C] and five moisture treatments [matric potential from -5 to -1200 kPa]) using three agriculturaiiy important New Zealand sous (sous with a iiistory of pasture, arabie, or vegetable cropping). Mineralization was iinearly related to gravimetric moisture content, except in the high-C pasture soil where Oj suppiy apparentiy iimited mineraiization at high (25°C) temperature-moisture combinations. Temperature responses were adequately described by a Q^Q function (Q-¡Q values for C mineralization ranged 1.9-2.8). The pooi of mineraiizabie C, estimated using a first-order kinetic modei, increased as temperature and moisture increased, whereas the rate constant did not show a consistent trend witii either temperature or moisture. Part of the C mineralized during tiie incubation was from the microbial biomass (post-incubation biomass C decreased by an average of 0.22-0.31 mg kg~^ for each 1 mg kg~^ CO2-C evolved). Microbiai biomass C (MBC) was particuiariy sensitive to temperature (post-incubation biomass C decreased 18-35% between 5-25°C).Tiie deciine in MBC between 5 and 12°C represented an average of 40% of the C mineraiization increase in tiiat temperature intervai. Between 18 and 25°C, the decline in MBC was equivalent to only 20% (on average) of tiie C mineraiized in tiiat temperature interval. High Q^Q vaiues reported in iaboratory incubations at low temperature may be partly due to mineraiization of microbiai C.Abbreviations: DOM, dissolved organic matter; MBC, microbial biomass carbon; SOM, soil organic matter. S oil organic matter (SOM) plays a key role in the global C and N cycles. Soils contain more than twice as much C as the atmosphere and three times the amount stored in living plants (Schlesinger, 1997). Key factors determining the levels of SOM are inputs of plant and animal residues and losses via decomposition. Decomposition (mineralization) of SOM is microbially mediated, with the rate of mineralization being strongly dependent on temperature and soil moisture. Stanford and his coworkers (Stanford and Smith, 1972;Stanford and Epstein, 1974;Stanford et al., 1973) advanced the concept of a potentially mineraiizabie pool of organic matter and a related mineralization rate constant to characterize the temporal pattern of N mineralization. There is wide acceptance that each soil has a fixed quantity of mineraiizabie N (and C), which can be experimentally determined using incubation assays (Campbell et al., 1993;Motavalli et al., 1994). In theory, potentially mineraiizabie C is the amount of C that will mineralize in infinite time. In practice, it is estimated by incubating soil und...

There is keen interest among soil scientists in identifying chemical assays that may be used as predictors of soil N mineralization potential. Our objective was to determine if hot water‐extractable N (16‐h extraction at 80°C) is a useful predictor of mineralizable N and plant N availability. In a group of 30 New Zealand soils, representing different management histories and parent materials, hot water extracted between 2.6 and 8.7% of total N. The extracted N consisted mainly (∼80%) of organic N, with the remainder being NH4–N, generated by hydrolysis of heat‐labile organic N. The C/N ratio of the extracted organic matter was relatively low (mean 8:1 vs. 11:1 for total organic matter), indicating that it included N‐rich substrates (i.e., substrates likely to have high mineralization potential). However, about three‐quarters of the extracted organic N was relatively recalcitrant, i.e., it did not hydrolyze to ninhydrin‐reactive N (NH4–N, amino acid‐N, amino sugar N) when treated with 1 M HCl for 6 h at 80°C. The contribution of mineralized N to plant N uptake was measured using a greenhouse‐grown oat (Avena sativa L.) crop, which received no added N. Hot water‐extractable N accounted for 50% of the variation in plant N derived from mineralization (PNDM), compared with 16% for total soil N, 32% for anaerobically mineralizable N (AMN), and 24% for NH4–N released by hot 2 M KCl. The best predictor of PNDM was N mineralized in a 28‐d aerobic incubation at 20°C (79% of variability in PNDM explained).