Biogeochemical Cycling of Carbon and Nitrogen in Rainfed Rice Production Under Conventional and Organic Rice Farming

Dwindling carbon (C) and nitrogen (N) levels in paddy soils decreases rice production and threaten human food security globally. The efficient maintenance of C and N fluxes in soil-rice systems is a crucial prerequisite for agricultural and environmental sustainability. Herein, we examined the C and N fluxes from 63 rainfed rice paddy fields under conventional farming (CF) and organic farming (OF) systems in Thailand. The C and N fluxes were measured based on a detailed analysis of relevant influxes (fertilizer, manure, and biomass addition) and effluxes (biomass harvest and greenhouse gas emission). The result demonstrated that the harvested grain and straw contributed to the most abundant C and N effluxes for both farming systems. The CH4 effluxes were moderate, whereas the N2O effluxes were meager relative to their total effluxes. Stubble incorporation and animal manure addition to soil were the most extensive C influxes. However, the primary N influxes were stubble incorporation and animal manure addition for the OF system, and chemical-N fertilizers for the CF system. Net C depletions were observed in both the CF and OF systems. However, net N was depleted and accumulated in the CF and OF systems, respectively. Straw incorporation to soils could restore the net C accumulations for the CF and OF systems and elevate the net N accumulation for both systems. This study highlighted that complete straw removal has exacerbated the C and N stock in soil-rice systems, inducing insecurity for the environment and the agricultural systems. Effective straw management is a simple approach for sustaining paddy rice production

Most Plant Nutrient Elements Are Retained by Biochar in Soil

Biochar may contain substantial amounts of plant nutrient elements, and at typical rates of application, may supply luxury levels of K, Ca, P, and other plant nutrients. However, little is known of the agronomic effectiveness of these nutrients because they exist in diverse compounds and are located in the microporous matrix of biochar particles. We have identified the compounds and location of nutrient elements in three biochars and observed their release from biochar particles in soil. Much K was quickly released from biochar but little or no Ca, Mg, S, and P were released over eight months, which represents a very different behavior from chemical fertilizers that are mostly water soluble. There is clearly a need to determine the availability to plant nutrients in biochar. Appropriate laboratory methods should be developed for measuring the availability of plant nutrients as standard methods of fertilizer analysis are ineffective

Speciation and pH- and particle size-dependent solubility of phosphorus in tropical sandy soils

Growing demands for crops have expanded agricultural production to sandy soils with low available phosphorus (P) content and high P mobility. Knowledge of P speciation and solubility in such soils is poorly understood. Here we investigated: i) the P speciation in the clay size fraction of sandy soils, ii) the pH-dependent solubility of soil orthophosphate (PO4–P), and iii) the particle size-dependent solubility of the soil P. The studied loamy sand soils were low in organic matter (2.6–7.9 g kg−1) and deficient in available P (1.5–8.5 mg kg−1), and kaolinite (21%) was the only clay mineral identified using X-ray diffraction technique in the clay-sized particles. The P K-edge X-ray absorption near edge structure analysis revealed that P associated with Al phases, 45% adsorbed to gibbsite and 35% in variscite (AlPO₄·2H₂O), dominated the P speciation in the clay-sized particles. The very low PO4–P solubilities in the investigated bulk soils exhibited a pronounced positive pH-dependent pattern that was consistent with the pH-dependent solubility of Al3+ and Fe3+. Geochemical modeling suggested that the maximum PO4–P solubilities were linked to the solubility of variscite and strengite (FePO₄·2H₂O), indicating that PO4–P solubilities were controlled by the solubility of Al3+ and Fe3+ as restricted by the pH-dependent solubility of Al phases and Fe (hydr)oxides. The analysis of particle size fractions revealed that the largest size fraction (1000–2000 µm), containing visible plant debris and contributing the least to total soil mass (<0.126%), contained most of the total soil P stocks (50–60%). The small size factions (<250 µm) were the key to available PO4–P. Our data highlighted the pivotal role of Al/Fe (hydr)oxides and kaolinite in controlling the pH-dependent solubility of PO4–P in these tropical sandy soils having very limited contents of total and available P and a high risk of P loss.ISSN:0016-7061ISSN:1872-625

Effects of Biochar on Properties of Tropical Sandy Soils Under Organic Agriculture

International audienceThis study evaluated the influences of biochar made from local agricultural wastes on sandy soils in farmer fields where biochar has been used as a soil amendment for more than three years. The major objective of this study was to gain insight into the effects of long-term biochar application on properties of sandy soil. Unamended soil properties were compared to biochar-amended soils properties using the paired samples t-test (p &lt; 0.05). The statistical results of the study indicated that cation exchange capacity, exchangeable potassium, available phosphorus, field capacity, plant available water, water-stable aggregate size fractions (&gt; 1 and &lt; 0.25 mm), median aggregate size and aggregate stability were significantly different at p &lt; 0.05. Clearly, biochar present for 3 or more years can improve soil physicochemical properties. We conclude that sandy soil properties, especially soil physical properties, are very strongly affected by biochar application combined with conservative soil management. Biochars affect both physical and biological mechanisms of soil aggregate formation because the biochar particle sizes influence the arrangement of clay on biochar and biochar grains provide a favorable microbial habitat and food source for fungi creating microorganism-biochar-soil associations which enhance water-stable aggregates and water holding capacity

Combining quantitative (palaeo-)pedological, palaeo-environmental studies and modelling: an important step on the way to predict soil reactions to environmental change

A study on a Holocene soil chronosequence in S-Norway is used to test the capability of the model SoilGen to model the development of soils with clay illuviation. SoilGen models soil formation as a function of the soil forming factors. Thus, the latter had to be reconstructed for the time span of soil development. The factors ‘relief’ and ‘parent material’ were obtained by field and laboratory analyses, the factor ‘time’ was derived from existing sea level curves, and the factors climate and organisms were obtained from literature and from a recent palaeo-environmental study. The chronosequence has been established on loamy marine sediments, and shows Albeluvisol development with time. Clay illuviation starts within 1650 years. The characteristic albeluvic tongues start to form after 4600 to 6200 years. They develop preferably along cracks. Albeluvic material falls into the cracks, leading to the development of albeluvic tongues, which become deeper and wider with time. Development of pH, CEC and clay content with time as measured in the investigated pedons is compared with the model results in order to check, to which degree model results agree with observed results