CONTENTS I . Introduction . 11. Factors which influence microbial biomass . . . ( I ) Soil chemical factors . . . . . . . ( a ) Carbon and nitrogen limitation . . . . ( b ) Residue and nutrient amendment . . . (c) Influence of plants . . . . . . ( d ) Soil pH . . . . . . . . (e) Heavy metals and pesticides . . . . (2) Soil physical factors . . . . . . . ( a ) Soil texture and structure ( b ) Soil moisture and temperature (c) Seasonal fluctuations . . . . . . 111. Global distribution of microbial biomass . . . ( I ) Relationship with soil chemical factors and macroclimate

A comparative assessment of factors which influence microbial biomass carbon and nitrogen levels in soil

Organic inputs to the soil must be considered separate from soil organic matter (SOM) to understand properly the physical, chemical, and biologi­ cal processes involved in the sustainability of tropical agroecosystems. Present management of organic inputs is largely empirical and qualita­ tive. A better understanding of the role ot organic inputs on the processes involved in litter decomposition, nutrient cycling, and soil aggregation is needed. The management of organic inputs should be as predictable as that of inorganic inputs, such as fertilizers and lime. The application of or­ ganic inputs should promote the synchrony of nutrient release w;th plant growth demands. Predictive parameters are needed to approach synchrony. Research on decomposition of organic inputs should be con­ ducted in various agroecosystems. Very little is known about the role of root litter in tropical agroecosystems. Six research imperatives are iden­ tified for tropical agroecosystems: (1)quantify the biomass and nutrient content of aboveground organic inputs: (2)develop predictive parameters for nutrient release patterns, or quality, of organic inputs; (3) investigate the effect of piacement of organic inputs on nutrient Organic Input Management in Tropical Agroecosystems 125 availability; (4) quantify production and nutrient releaie by roots, identify resource quality parameters (see Chapter 4) that describe root decom­ position, and determine the relative importance of roots as sources of nutrients: (5) investigate the effects of quality and placement of organic in­ puts on soil physical properties; and (6) understand how organic inputs are transformed into functional SOM pools in soils differing in texture, mineralogy, and moisture and temperature regimes. We need reliable field methods ior measuring root production and decomposition, transfer from organic inputs into SOM fractions, and transfer processes among SOM pools. The diversity of tropical species and farming systems provides ample combinations of crops and organic inputs for improving tropical agroecosystems. Agroecosystems differ from natural ecosystems in that large amounts of biomass and nutrients are removed as crops. Nutrient outputs via crop harvests far exceed the other nutrient loss pathways combined. This is shown in Table 1 where asummary of annual crop nutrient budget studies from temperate-region countries is presented. A long-standing principle of sound agriculture is to replace the nutrienis lost via crop harvests by adding either inorganic or organic fertilizers. Aside from adding nutrients, organic additions improve soil physical properties and help maintain soil organic matter content, all of which positively affect plant productivity (Allison, 1973). Organic inputs to the soil consist of aboveand belowground litter, crop residues, mulches, green manures, animal manures, and sewage. Some of these inputs are grown and recycled on site " Tablo 1. Summary of nutrient inputs and outputs (kg hayr") of 18 crop production systems from the United Statos, United Kingdom, Netherlands, France, Israel, and Japan. (Mean and standard deviations calculated from data of Frissel, 1978.) Nutrient inouts or outouts Nitrogen Phosohorus Potassium Inputs Inorganic fertilizer 156 t 189 39 "t38 119 = 188 Nfixation 19 ±: 39 Other 13 ±: 13 0.4 t 0.8 9 13 Total 188 ± 75 39 "t39 127 t 176 Outputs -arvest removal 103 t 86 16 ±16 91 ±t 13 Denitrification 20 224a Leacflng 38b nd nd Total 127 ±: 95 16 = 6 91 ± 13 Balance 60 "93 24 ±30 37 = 77 a. Mean of 10 out ot 18 data sets reportir,j denitrification. b. Mean of 3 out of 18 data sets reporting leaching. nd = not determined. 126 Dynamics of SOM in Tropical Ecosystems (internal to the system), while others are brought in from other sites, thu, constituting an cetrnal source of nutrients and orezanic C. Organic inputs are distinct from soil organic matter (SONM), which can be defincd as organic material of biological oricin that has undergone partial or complete transformation in the soil and is located below the soil surface. In much of the literature, organi.: 'puts and SOM arc lumped together as "organic matter," creating considerable con-fusion %4hcnattempting to evaluate and manage the role of organic :nputs c mpared to that of sod organic matter. We proposc, therefore, to discourai'c using the term organicmatterand encourage the use of organic inputs and soti ,rgantc matter ISOM). Although early acriculturc depcnded heavily on organic inputs, the emphasis shiftcd to inorganic fertilizcrs as they became abundant and economi­ cally practical. Today, because agricultural development in much of the tropics is largely limited by economic constraints, emphasis must again be placed on use of organic inputs along 'ith chcmic:;l fertilizers. This paper briefly reviews Figure 1. Conceptual model cf ra;cr -ccls and transfers of soil organic matter. Numbers indicate focus of research 'mperatives (see text conclusions).

Dynamics of Soil Organic Matter in Tropical Ecosystems

A grassland soil was incubated for 240 days at either 15. 25 or 35°C. Changes in soil microbial biomass were monitored throughout the incubations in three ways: from soil ATP content, by the fumigation-extraction method and by a new version of the fumigation-extraction method in which carbohydrate released by fumigation is measured, rather than total C. All three methods of measuring biomass concur in showing that the biomass drifted downwards very slowly at 15° (with a mean decline of 0.11% per day) and 25°C (0.21% per day), whereas it fell precipitously during the first 50 days of incubation at 35°C (1.72% per day over the 0–50 day period). Both CO 2 output and biomass C remained remarkably constant over the 150–240 day period in all the incubations. The specific death rates of the biomass over this period were 0.0072, 0.016 and 0.25 day −1 at 15. 25 and 35°C respectively. The corresponding biomass turnover-times were 139, 62 and 4 days. Raising the incubation temperature had two effects: the rate of release of substrate increased but the specific death rate increased even more. The activation energy of death was 130kJ mol −1 , similar to values for thermal death in bacteria, suggesting that thermal denaturation was the process responsible for the enhanced death rate at 35°C. Although some of the CO 2 liberated during these prolonged incubations must have come from the initial microbial biomass, through cryptic growth and consumption of intracellular C reserves, most can only have come from soil organic matter. Some chemical or biochemical process (or processes) must operate on soil organic mailer to release a steady flow of substrate to the waiting biomass. The measurement of carhohydrate released by fumigation may provide a convenient way of measuring soil microbial biomass and deserves further testing.

Survival of the soil microbial biomass at elevated temperatures

Four biochar types, produced by slow pyrolysis of poultry litter (PL) and pine chips (P) at 400 or 500 °C, were added to two adjacent soils with contrasting soil organic matter (SOM) content (8.9 vs. 16.1 g C kg−1). The N mineralization rate was determined during 14-week incubations and assessments were made of the microbial biomass C, dehydrogenase activity, and the microbial community structure (PLFA-extraction). The addition of PL biochars increased the net N mineralization (i.e., compared to the control treatment) in both soils, while for treatments with P biochars net N immobilization was observed in both soils. Increasing the pyrolysis temperature of both feedstock types led to a decrease in net N mineralization. The ratio of Bacterial to Fungal PLFA biomarkers also increased with addition of biochars, and particularly in the case of the 500 °C biochars. Next to feedstock type and pyrolysis temperature, SOM content clearly affected the assessed soil biological parameters, viz. net N mineralization or immobilization, MBC and dehydrogenase activity were all greater in the H soil. This might be explained by an increased chance of physical contact between the microbial community activated by SOM mineralization upon incubation and discrete biochar particles. However, when considering the H soil's double C and N content, these responses were disproportionally small, which may be partly due to the L soil's, somewhat more labile SOM. Nonetheless, increasing SOM content and microbial biomass and activity generally appears to result in greater mineralization of biochar. Additionally, higher N mineralization after PL addition to the H soil with lower pH than the L soil can be due to the liming effect of the PL biochars.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/gcbb.12119

Biochar amendment to soils with contrasting organic matter level: effects on N mineralization and biological soil properties

A number of approaches have been used to model and thus describe N mineralization kinetics in soils. In this paper, we review and present a comparative evaluation of some approaches to modelling adopted for this purpose. These may broadly be categorized as: (1) simple functional approaches to predict net N mineralization and, (2) mechanistic approaches that include a description of microbial biomass processes to predict long-term C and N turnover in soils. The following conclusions emerged. First, the simple functional models try to quantify one or more active fractions of organic matter with associated rate constants to predict net N mineralization. A minimum of two organic N fractions are considered essential to significantly contribute towards N mineralization. Second, the mechanistic models have the advantage that model algorithms represent the basic mechanisms believed to influence mineralization. The input parameters of the models could be obtained independently of the test data. However, it is difficult to validate the models as some of the presumed functional pools cannot be quantified by physical, chemical and biological techniques. Consequently, the models have to be calibrated by adjusting the rate coefficients and pool sizes (site-specific) to fit the measured data. Generally, the performance of a model depends on site-specific calibration. In future, a major challenge will be to free simulations from site-specific calibration and to devise experimental methods for providing initial values to run a model. The amount of N mineralized during laboratory incubation experiments and the parameter estimates for different models depend on methods used to pretreat the soil prior to incubation and their exposure to experimental conditions (such as temperature, soil moisture content) during the incubation. To obtain comparable values amongst studies it is, therefore, necessary to standardize methods of sample pretreatment as well as incubation conditions.

A critical review of some approaches to modelling nitrogen mineralization

Net carbon and nitrogen mineralization rates were determined for an arable soil during 12 weeks at 37†C using an aerobic incubation-leaching technique. The amounts of mineralized C and N were compared to changes in the contents of C and N in microbial biomass (as determined by the chloroform fumigation incubation method; CFIM) during the incubation and to amounts of organic C and N in the leachates. Microorganisms were also followed by direct counting of bacteria, measurements of total hyphal lengths and fluorescein diacetate (FDA)-active hyphae, and by most probable number determinations of protozoa (naked amoebae and flagellates). Numbers of naked amoebae increased nearly 10-fold initially and then decreased between weeks 6 and 12. Bacterial numbers and FDA-active hyphae decreased during the incubation, and the relative composition changed slightly in favour of bacteria. Total hyphal lengths remained almost constant. A total of 105 μg N g' − soil dry wt and 1179 μg C g − soil dry wt was mineralized during the incubation, while the microbial N pool decreased by 42 γm − soil dry wt and the microbial C pool decreased by 225μ g − soil dry wt. Soluble organic matter in the leachates amounted to 16 and 31% of mineralized C and N, respectively. The possibility of measuring C mineralization with less frequent teachings and determinations of N mineralization offers an easy method for assessing changes in labile soil organic matter over time or for comparisons between soils. Through the use of appropriate C-to-N ratios, the N-content in the labile pool can be calculated.

Microbial biomass in relation to c and n mineralization during laboratory incubations

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Microbial biomass in relation to c and n mineralization during laboratory incubations