Dinesh K. Benbi

Carbon and Nitrogen in the Terrestrial Environment

Introduction.- 1. Carbon and Nitrogen Pools in Terrestrial Ecosystems.- 1.1 Forms and quantities of carbon and nitrogen on earth.- 1.2 Carbon and nitrogen in terrestrial phytomass.- 1.3 Carbon and nitrogen in soils.- 1.4 Global vegetation-soil organic matter interrelationships.- 2. Carbon and Nitrogen Cycles in Terrestrial Ecosystems.- 2.1 The global carbon cycle.- 2.2 The global nitrogen cycle.- 2.3 Carbon and nitrogen cycling in soils.- 2.4 Global climate change and C and N cycling.- 3. Soil Organic Matter Characterization.- 3.1 Chemical characterization of soil organic matter.- 3.2 Physical characterization of soil organic matter.- 3.3 Morphological characterization of soil organic matter.- 4. Organic Matter and Soil Quality.- 4.1 Soil quality.- 4.2 Impact of SOM on soil physical, chemical and biological properties.- 4.3 Evaluation of organic components as soil quality indicators.- 4.4 Use of combined biological parameters for soil quality estimation.- 5. Carbon and Nitrogen Transformations in Soils.- 5.1 Transformations of organic components.- 5.2 Transformations of inorganic components.- 6. Anthropogenic Activities and Soil Carbon and Nitrogen.- 6.1 Land use changes.- 6.2 Agricultural management.- 6.3 Ecosystem disturbance.- 7. Leaching Losses and Groundwater Pollution.- 7.1 Dissolved organic carbon.- 7.2 Dissolved organic nitrogen.- 7.3 Nitrate leaching.- 8. Bidirectional Biosphere-Atmosphere Interactions.- 8.1 Atmospheric Nitrogen Depositions.- 8.2 Carbon fixation via photosynthesis.- 8.3 Biological N2 fixation.- 8.4 Carbon dioxide emission.- 8.5 Methane emission.- 8.6 Emission of oxides of nitrogen: N2O and NO.- 8.7 Ammonia emission.- 8.8 Global climate change and crop yields.- 8.9 Economics of carbon sequestration.- 9. Modelling Carbon and Nitrogen Dynamics in the Soil-Plant-Atmosphere System.- 9.1 Carbon dioxide exchange from soils.- 9.2 Methane emission from rice fields and natural wetlands.- 9.3 Nitrogen trace gas emission.- 9.4 Modelling nitrogen dynamics in soils.- 9.5 Modelling organic matter dynamics in soils.- References.- Index.

A critical review of some approaches to modelling nitrogen mineralization

Abstract. 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.

Sustainable yield trends of irrigated maize and wheat in a long-term experiment on a loamy sand in semi-arid India

Long-term field experiments play an important role in understanding the complex interactions of plants, soils, climate and management and their effects on sustainable crop production. A long-term fertilizer experiment with maize-wheat-cowpea (fodder) is in progress since 1971 at Punjab Agricultural University farm Ludhiana, India. The experimental result for the first 21 years showed that application of N alone or in combination with P did not produce as much maize and wheat grains as the application of N, P and K together. Eight years after the start of the experiment, the optimal levels of N, P and K application (100% NPK) were unable to sustain the similar (maize) yield level as before because of Zn deficiency. Whereas in FYM amended plots the Zn deficiency did not appear and the higher crop yields could be sustained. The chemical control of weeds could not sustain the maize productivity at the same level as the manual removal of weeds. It was concluded that the high level of crop production can be sustained with the application of N, P and K under intensive cropping system provided deficiency of any of the micronutrient does not crop up. The deficiency of Zn is most likely to occur in semi-arid light textured alluvial soils under intensive cropping without the addition of farm yard manure/organic manures. In maize based cropping systems, manual control of weeds may be preferred to the chemical one. Addition of FYM in conjunction with 100% NPK is most beneficial both from bio-physical and economic point of view.

Nitrate distribution and accumulation in an Ustochrept soil profile in a long term fertilizer experiment

Distribution and accumulation of NO3—N, down to 210 cm depth, in the soil profile of a long term fertilizer experiment were studied after 16 cycles of cropping (maize-wheat-fodder cowpea). The application of fertilizer N without P and K or in combination with only P resulted in higher NO3—N concentration in the soil profile than the application of N with P and K. With an annual application of 320 kg N ha−1 alone, a peak in NO3—N accumulation occurred at 135 cm soil depth. However, with the application of NPK, no peak in NO3—N distribution was discernible and its content at most of the sampling depths was either less than or equal to N and NP treatments. The annual application of 10 tons farm yard manure (FYM) per ha along with NPK resulted in a relatively lower NO3—N content in the sub soil. The amount of NO3—N accumulation in the soil profile decreased as the cumulative N uptake by the crops increased. Application of fertilizer amounts greater than that of the recommended (100% NPK) resulted in low percent N recoveries in crops and greater NO3—N accumulation in the soil profile.

Soil carbon pools under poplar-based agroforestry, rice-wheat, and maize-wheat cropping systems in semi-arid India

Labile fractions of soil organic C are considered important indicators of soil quality as these can respond rapidly to land-use changes and agricultural management. We studied the impact of three different land-use systems viz. poplar-based agroforestry involving wheat-legume rotation, rice-wheat and maize-wheat agroecosystems, on dynamics of total organic C (TOC), oxidisable soil organic C (SOC), very labile, labile, less labile, and recalcitrant C fractions, water extractable organic carbon (WEOC), hot water soluble C (HWC), microbial biomass C (MBC), and mineralizable C in the semi-arid subtropical India. The maize-wheat and agroforestry systems had 65–88% higher SOC stocks than the rice-wheat system and were characterized by predominantly labile C. About 56–60% of the total organic C in maize-wheat and agroforestry systems occurred as labile and very labile C compared to 37% under rice-wheat rotation. Contrarily, the majority of organic C (63%) in rice-wheat soils was stabilized in less labile and recalcitrant forms. The HWC and MBC were also higher in maize-wheat and agroforestry systems as opposed to the rice-wheat system. In the discriminant function analysis, a composite of indicators involving TOC, recalcitrant C and total N correctly distinguished the soils under the three systems. The results suggested that in agroforestry and maize-wheat systems the organic C in soils was less stable and thus could be lost following the land-use change.

Sensitivity of Labile Soil Organic Carbon Pools to Long-Term Fertilizer, Straw and Manure Management in Rice-Wheat System

Abstract Labile soil organic carbon (SOC) pools, estimated through chemical fractionation techniques, are considered sensitive indicators of management-induced changes in quality and composition of soil organic matter. Although the impacts of organic manure and crop residue applications on C sequestration in rice-wheat system are fairly well documented, their influence on labile SOC pools is relatively less known. Impacts of organic manure, rice straw, and inorganic fertilizer nitrogen (N) applications on soil total organic carbon (TOC) and SOC pools including water-extractable organic C (WEOC), hot water-soluble organic C (HWOC), potassium permanganate-oxidizable organic C (KMnO4-C), microbial biomass C (MBC), mineralizable organic C (Cmin), and the oxidizable fractions of decreasing oxidizability (easily-oxidizable, oxidizable, and weakly-oxidizable) were investigated in an 11-year field experiment under rice-wheat system. The field experiment included treatments of different combinations of farmyard manure, rice straw, and fertilizer N application rates, with C inputs estimated to be in the range from 23 to 127 Mg ha−1. After 11 years of experiment, WEOC, HWOC, and KMnO4-C were 0.32%–0.50%, 2.2%–3.3%, and 15.0%–20.6% of TOC, respectively. The easily-oxidizable, oxidizable, and weakly-oxidizable fractions were 43%–57%, 22%–27%, and 10%–19% of TOC, respectively. The applications of farmyard manure and rice straw improved WEOC, HWOC, KMnO4-C, easily-oxidizable fraction, Cmin, and MBC, though the rates of change varied considerably from –14% to 145% and –11% to 83% of TOC, respectively. At the C input levels between 29 and 78 Mg C ha−1 during the 11-year period, the greatest increase was observed in WEOC and the minimum in KMnO4-C. Water-extractable organic C exhibited a relatively greater sensitivity to management than TOC, suggesting that it may be used as a sensitive indicator of management-induced changes in soil organic matter under rice-wheat system. All the other labile SOC pools exhibited almost the same sensitivity to management as TOC. Most of the SOC pools investigated were positively correlated to each other though their amounts differed considerably. Long-term applications of farmyard manure and rice straw resulted in build-up of not only the labile but also the recalcitrant pool of SOC, emphasizing the need for continued application of organic amendments for permanence of the accrued C under the experimental conditions.

Nitrogen balance and N recovery after 22 years of maize-wheat-cowpea cropping in a long-term experiment

The influence of N, P and K application through inorganic and organic fertilizers on N recovery in crop plants and its balance in the soil-plant (maize-wheat-cowpea fodder) was studied for the first 22 years of a long-term experiment at Punjab Agricultural University farm, Ludhiana, India. The results showed. that N removal and apparent N recovery by both maize and wheat was directly related to the balanced application of N, P and K fertilizers. Averaged over the years, application of N alone (100% N) resulted in a recovery of 17.1% in maize and 31.7% in wheat. The application of P and K along with N almost doubled (32.8% in maize and 64.7% in wheat) the apparent N recovery in the crops. Increase in soil N concentration which was related to the build-up of soil organic carbon (OC) occurred at a very slow rate with the application of N, P and K fertilizers. Addition of farm yard manure (FYM) resulted in highest N removal in crops and build-up of soil N and OC status. Application of recommended N without P and K fertilizers resulted in relatively large amounts (64–71%) of fertilizer N lost from the surface soil as compared to that (41–49%) with N, P and K applied together. Higher rate of fertilizer application (150% NPK) resulted in comparatively greater N loss (58–62%). It was concluded that balanced and judicious use of N, P and K fertilizers coupled with the addition of any deficient element (e.g. Zn) help in minimizing N losses and environmental pollution.

Modeling of nitrogen transformations and translocations

Different submodels within complex model packages on N regimes-for plant N-uptake, net N-mineralization, nitrate leaching and microbial N immobilization-are critically reviewed mainly with regard to their prediction ability on the basis of three comparative papers. Only for some of the processes adequate statistical evaluation of the models was possible. Compared to the other statistically evaluable process, nitrate leaching, modeling of plant N-uptake yields the better results. Most models for mineralization use arbitrary approaches rather than empirical ones. Although only approximate estimates of N mineralisation were at hand, the models generally behave expectedly poor. Only one model-DAISY-out of 16 involved in the comparison uses an explicit microbial biomass sub-model including microbial growth, decline and maintenance terms. So DAISY is the only model coupling C and N cycles. But what is true for an individual model describing the C and N transformation of a lab incubation experiment seems to be valid for most of the complex simulation work on the C and N regimes: this model was said to be overparameterized with respect to the available data.

Modeling Nitrogen Mineralization Kinetics in Arable Soils of Semiarid India

Laboratory incubation experiments were conducted to study N mineralization kineticsin 15 benchmark soils of Punjab, India. Aerobic incubations were carried out for 112 days at 32°C and −33 kPa water potential. Cumulative N mineralization in different soils ranged from 8.2 to 75.6 mg kg −1 soil, which constituted 2.7 to 8.8% of soil organic N. For each gram of organic carbon (OC) kg −1 soil, about 9 mg N was mineralized in 112 days of incubation. Mineralization was faster during initial days of incubation followed by a relatively slower rate subsequently. Kinetic models of varying complexity were fitted to N mineralization vs. time data. The first-order single compartment (FOSC) model provided relatively poor fit to the data and systematically deviated from the measured values. Both the first-order double compartment (FODC) and the first-order zero-order (FOZO) models provided good fit to the data and yielded very low residual mean squares. In soils with OC content > 0.55%, the FODC model invariably provided the best fit. It was concluded that a minimum of two pools of organic matter may be considered to contribute towards organic N mineralization in these soils. Soil OC and clay strongly influenced the FOSC- and FODC-predicted N mineralization potential of soils. A linear regression involving OC and clay × OC interaction could be used to predict N mineralization potential of the soils.

Using Carbon Management Index to Assess the Impact of Compost Application on Changes in Soil Carbon after Ten Years of Rice–Wheat Cropping

Soil organic carbon (C) is a major determinant for the sustainability of agricultural systems. The changes in C pools (active or total) reflect the changes in an agricultural system. The C management index (CMI) can be used to monitor the soil over time, and it also tells whether a new system or practice is declining or rehabilitating the soil. Carbon management index was calculated for a long‐term experiment after 10 cycles of rice–wheat cropping to assess the influence of rice straw compost application either alone or in combination with inorganic fertilizers on soil C buildup. Total and labile C was greater in rice straw compost–amended soil as compared to unamended control or soils receiving inorganic fertilizers only. Application of rice straw compost increased the mean CMI (47.1) as compared to control (21.0). Labile C was positively related to mean weight diameter, and seemingly it plays an important role in the maintenance of physical fertility of soils and thus sustainability of the cropping system.