Microbial biomass nitrogen (MBN)

Microbial Biomass N is the assimilated N in the body of the microbes. It is in a state of dynamic equilibrium and represents a significant proportion of the total soil N, which is relatively constant. The biomass N increased from 46 pg N in control to 80 pg N g"1 soil by 5th day and remained at this level at 20th day on application of labeled wheat straw to a clay loam soil (Ocio et al. 1991). Bremer and Van Kessel (1992) worked on the microbial C and N dynamics after incorporating 14C- and 15N-labeled lentil and wheat straw under field conditions in a sandy loam soil. In their experiment, 65 to 81% of added 15N was traced in residue which suggested that microbial biomass minimizes losses of N during the lag phases of crop demand and may supply the same during the log phase. Kushwaha et al. (2000) showed increase in MBC from 214-264 pg g"1 during crop growth from in straw removal treatment and from 368-503 pg g"1 in straw incoiporation treatment after two aimual rice-barley crop cycles. The MBC and MBN were enhanced by 48% and 60%, respectively, in residue retained over residue devoid plots. Microbial biomass may behave as slow- release N fertilizer. Bird et al. (2001) observed significantly higher soil microbial biomass on straw incorporated plots compared to straw devoid plots. As soil microbial biomass is a major source of plant available N, straw incoiporation increased the soil N status as well as humic substances.

Potentially mineralizable N

Potentially mineralizable nitrogen (PMN) is an important biological indicator which shows the capacity of a soil microbial community to mineralize nitrogen present in soil organic residues into one of the plant available forms of N, i.e., NH4+. Mijangos et al. (2006) studied the sensitivity of soil biological parameters, viz. soil enzymes, potentially mineralizable nitrogen, soil respiration, abundance of earthworms, and microbial community metabolic profiles on the fertilization and tillage practices in a field trial with forage com. They observed higher biological efficiency in organically fertilized, no-tillage plots, compared to conventional tillage and mineral fertilization. The values of potentially mineralizable N, basal respiration, substrate induced respiration, and earthworm abundance thus recorded were 23.1, 35.3, 55.7, and 226.9 per cent more in organically fertilized, no-tillage plots than in inorganically fertilized, conventional tillage plots, respectively. They concluded that potentially mineralizable nitrogen as well as soil enzymes play a key role as early indicators of soil quality changes compared to conventional physicochemical parameters as they were more sensitive to monitor the relatively subtle changes in soil properties caused by these management practices.

Enzyme activities

Enzymes are the biochemical agents that catalyze the soil metabolic processes. In the past, majority of researches focused on the microflora in soil and hardly any focus was put on the extracellular enzymes of microbial origin formed after decomposition of organic matter. The principal functions of enzymes in soil include decomposition of organic matter, catalyzing metabolic reactions, assisting various life processes of microorganisms in soils, stabilization of soil structure, humus formation, nutrient cycling, facilitating an early indication of the soil history and effects of agricultural management (Ceron and Melgarejo 2005, Kandeler et al. 2006). For these, they are used as soil biological indicators from the 80s. Due to their intimate association with several indicators of biogeochemical cycles, decomposition of organic matter and soil remediation processes, they can be used to assess the physical, chemical and biological properties and therefore the quality of a soil (Gelsomino et al. 2006). Enzymes are widely recognized as good indicators because of their (a) close relation to organic matter decomposition, soil physical characteristics, microbial activity and biomass in the soil, (b) provide early information about changes in soil quality, and (c) rapid assessibility (Dick 1996, Nielsen and Winding 2001, Eldor 2007). It is due to the fact that enzymes in soil are produced both intra and extracellularly from microbes (bacteiia, fungi, plants, and a range of macro invertebrates) and are associated to different substrates like alive or dead cells, clays or/ and humic molecules and assay laboratory' conditions, it is essential to optimize the protocols for enzymatic activity determination. Since extraction of enzymes from soils is a difficult task and they lose their integrity easily (Gianfreda and Ruggiero 2006, Verchot and Borelli 2005, Dick 1996), utmost care should be taken regarding temperature, incubation period, pH buffer, ionic strength of the solution, and substrate concentration before their assessment (Gutierrez et al. 2008, Eldor 2007).

5.7.1 /3-GJucosidase

It is widely observed in soil system and its presence has been detected in soil, fungi and plants. It has been used as a key soil quality indicator due to its crucial role on cellulose degradation, releasing glucose as energy source for the maintenance of metabolically active microbial biomass in soil (Dick 1996, Sotres 2005). It also helps in releasing energy upon degradation of labile carbon in soil thereby stabilizing recalcitrant C pools (Knight and Dick 2004). However, due to its reduced efficiency in the presence of heavy metals, it is a less advocated indicator of soil quality in heavy metal contaminated soil (Makoi and Ndakiderni 2008). In free state in soil system, it usually has a small lifespan, as it is easily prone to degradation, denaturation and irreversible inhibition. A portion of these free enzymes get adsorbed on soil clay minerals, gets incorporated with the humic particles and therefore lose stability and stay in soil in less catalytically active state (Marx et al. 2005, Bums 1982).

5.7.2 Phosphatase

Phosphorus is a primary nutrient crucial for plant growth (especially roots) and activation of several enzymes in plant system. The availability of this element is largely governed by soil pH. In acid soils, it is fixed as Fe-P and Al-P and in calcareous soils as Ca-P, thereby making it bio-unavailable (Dick et al. 2000). These fixed P are solubilized by soil microbes, e.g., phosphate solubilizing bacteria, viz. Bacilluspolymyxa, Pseudomonas striata, fungi, viz. Aspergillus awamori, PeniciUium digitatum. They release low molecular weight organic acids, which release the fixed P into soil solution through the production of extracellular enzymes as phosphatases (Sundara and Hari 2002). Phosphatases hydrolyze esters and anhydrides of phosphoric acid. Its activity depends on extracellular concentration of phosphatase enzymes, which can be in free state in soil solution, adsorbed in the humic fraction or clay minerals (Sundara and Hari 2002, Turner and Haygarth 2005). Since phosphomonoesterases are active both under acidic and alkaline soil reactions and solubilize low molecular weight P-compoimds, viz. nucleotides, sugar phosphates and polyphosphates, they are the most studied soil enzymes (Makoi and Ndakidemi 2008) and thus are widely used as soil quality indicators. A strong correlation was observed between phosphatase activity and soil properties such as pH, total N, organic P and clay content by Turner and Haygarth (2005) while working in temperate grassland, suggesting it to be a good soil biological indicator.

5.7.3 Dehydrogenase

Dehydrogenase is an important soil enzyme under oxidoreductase group which oxidizes a substrate through the reduction of an electron acceptor. In soil, determination of dehydogenase enzyme activity provides a large amount of information about soil fertility as well as soil health. These enzymes are an integral part of microorganisms and are instrumental in organic matter oxidation; nevertheless, its activity is weakly correlated with microbial respiration or microbial biomass (Dick 1996). In spite of this, it is considered as a soil quality indicator for its involvement in electron transport systems of oxygen metabolism in an intracellular environment (living cells) to show its activity (Kandeler and Dick 2007). Unlike hydrolases (p-Glucosidase, mease, phosphatase), it is inactive in extracellular environment and therefore cannot provide information about soil degeneration processes; its activity depends on management practices and/or climatic conditions (Kandeler and Dick 2007). Dehydrogenase activity is related to living and active cells, but presence of heavy metals hinders its activity through the catalysis of assay by extracellular phenol oxidase and alternative electron acceptors such as nitrate and humic substances (Tate 2002, Speir and Ross

  • 2002). Speir and Ross (2002) and Kandeler and Dick (2007) observed that presence of Cu adversely affected dehydrogenase activity.
  • 5.7.4 Urease

These enzymes hydrolyze urea into CO, and NH3 and consequently reduce soil pH through production of carbonic acid and hasten N losses through volatilization of gaseous NH3. This enzyme is of major concern as urea is the most widely used nitrogenous fertilizer in developing countries like India and the N availability to plants is regulated by this enzyme. However, since majority of N fertilizer is lost through volatilization and leaching, role of urease enzymes to the total losses of N is meager (Makoi and Ndakidemi 2008). Studies on other related enzymes, e.g., aimnonia monooxygenase (AMO), are in their infancy and sufficient data about their interaction in soil is still to be obtained (Gutierrez et al. 2009). For this reason, it is not included as a soil quality indicator but this membrane-bound enzyme could give insights on nitrification rates and the efficiency of nitrification inhibitors, instead of quantifying nitrate per se. Gutierrez et al. (2009) observed the spatial variability of AMO in a paddy field in Chile and repoxted its reduced activity in fallow and dry soils. However, a positive correlation was fouixd with soil available N suggesting its usefulness in studying nitrification, denitrification and volatilization processes in soil. Urease has widely been used as a soil quality indicator due to its sensitivity to management practices, especially organic fertilization, soil tillage, cropping history, organic matter content, soil depth, management practices, heavy metals as well as environmental factors, such as temperature and pH (Yang et al. 2006, Saviozzi et al. 2001). This enzyme, of extracellular oiigin, represents up to 63% of total soil enzymatic activity. Several authors have reported its relation to soil microbial community as well as soil properties (physical, chemical and biological) (Corstanje et al. 2007). The stability of this enzyme is dependent on organo-metallic complexes and humic substances, which makes them resistant to denaturing agents such as heat and other proteolytic scenarios. The better understanding of this enzyme will provide efficient ways to urea fertilizer management, especially in warm humid regions, submerged soils and under irrigated agriculture (Makoi and Ndakidemi 2008).

Species and population of arthropods

The abundance and diversity of invertebrate communities like arthropods are sensitive to chemical and physical soil characteristics. Soil biological quality is assessed through indices (Shaimon, Menhinick, Simpsonand Pielou indices). Santomfo et al. (2012) found Acarina, Collembola, Enchytraeids and Nematoda to be more resistant taxa to the urban environment. They reported collembolans to be more sensitive to changing soil properties. Maleque et al. (2009) found arthropod populations as sensitive bioindicators for the sustainable management of conifer plantations. The structural elements of forests (e.g., deadwood, coarse woody debris, understory vegetation, herbaceous plants, flowering plants, nectar plants, leaf litter) affect soil health which consequently influences the arthropod communities. They opined that monitoring arthropods as bioindicators may be a cost effective technique for assessing sustainable forest management plans.

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