土壤學(xué)基本知識(shí)-土壤有機(jī)質(zhì)性質(zhì)及其分解(編輯修改稿)
2025-02-13 20:21 本頁(yè)面
【文章內(nèi)容簡(jiǎn)介】 ents released from decay increases, plus those from dead microbes? excess nutrients released in mineral forms ? mineralization Sources of immobilized nutrients: 1) above soil surface, stem flow and throughfall are the major sources。 2) below ground, decaying soil OM。 3) free living microbes (particularly for N fixation) ?usually low in most terrestrial ecosystems. N and P tend to be immobilized mostly. Deposition of Litter and Soil OM5: About Nitrogen Dynamics (from 15N isotope studies): 1. N in litter is initially used by microbes 2. N in lignin increase during immobilization 3. Net release of N undetectable 13 indicate: a. Immobilization occurs in the early stage of decay。 b. High quality OM decay ? high N demand in growing microbial populations。 c. mineralization occurs only when lowquality OM remain ? microbial growth limited by C amp。 E. Deposition of Litter and Soil OM6: Significant immobilization can occur sorely chemically between litter and soil solution. In extremely poor sites, soluble OM in litter may be important in controlling nutrient balance. It is hypothesized that: petition for little available N ? plants produce leaf high in tannins ? release N in dissolved anic form ? mycorrhizae can take up. A shortcircuit N cycle ? reduce leaching loss of N DON NH4 Deposition of Litter and Soil OM7: In many terrestrial ecosystems, a potentially large fraction of weight loss in “decaying” litter can occur as leaching of soluble anics. The fate of this leached soluble anics is still unknown. Can we guess where these leached DOC go? Deposition of Litter and Soil OM8: N content in decaying OM plays diff. roles at diff. stages of decay: Shortterm study: high initial N content facilitate decay Longterm study: not quite so Two hypotheses: 1. Adding N to OM randomizes the bond structure in old litter/humus? make existing enzymes less effective。 2. Requirement for extracting N from humus to meet fungal N demands does not exist. (experiment shown that fungal shut down the production of lignin/humus degrading enzymes when large amount of N added) the system is messed up! Deposition of Litter and Soil OM9: Litter Decay Prediction: Litter nutrient quality determine short term decay rate, while litter carbon quality determines long term decay rate. Climatic conditions (summarized as AET) has a high correlations with litter decay. Deposition of Litter and Soil OM10: Litter Decay Prediction: … In addition, litter type (leaf, roots, woody materials have very different chemical positions) makes the picture more plicated. Soil animals play important roles in deposition (earthworms for example), but the details are less known. Deposition of Litter and Soil OM10: Humus production and deposition: Litter decay begins with a wide variety of materials of very diff. chemical quality and produces a much more homogeneous humus with lignin:cellulose ratio of ~1:1. To predict production of humus, modified equation % original remaining = ekt is used, but k is affected by initial litter N, Mg and Ca contents. Deposition of Litter and Soil OM10: Humus production and deposition: Turnover rate is used to evaluate humus deposition. Turnover rate can be expressed as a changing rate per unit time or the time period a pletely replacement required. Measuring methods (difficult to measure humus decay): 1. mesh bag method for soil OM weight loss? long time 2. field soil incubation for mineralization rate of N 3. 15N pool dilution analysis ? very high N immobilization 4. correlating soil respiration and gross N transformation 5. loss of total soil OM ? used in broad scale measurement Deposition of Litter and Soil OM11: Humus production and deposition: The methods give rath
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