Ro, Sophoanrith: Potential of organic manures in rainfed lowland rice-based production systems on sandy soils of Cambodia. - Bonn, 2016. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Sophoanrith Ro}},
title = {Potential of organic manures in rainfed lowland rice-based production systems on sandy soils of Cambodia},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2016,
month = apr,

note = {Rainfed lowland rice is the dominant food crop in the low-input agricultural systems of Cambodia. The main production area is characterized by sandy soils with low contents in nitrogen, phosphorus and organic matter, as well as low cation exchange capacity. The use efficiency of applied nutrients is reportedly very low, and the outcome of nutrient application strategies is highly variable. This study assesses the potential of organic manures to replace mineral N fertilizers in rainfed lowland rice-based production systems on sandy soils of Cambodia. It comprised field experiments and surveys. Four field experiments were conducted between 2013 and 2014 and differed by district and soil type (shallow vs. deep). Treatments compared the recommended rates of applied mineral N, with farmyard manure and mungbean (Vigna radiate) used as a pre-rice leguminous green manure. Legume treatments were further varied including with or without P application to the green manure and different residue management strategies (all residues returned, only grain harvested, all residues removed). The survey investigated farmers’ perceptions regarding potentials and constraints to leguminous green manure adoption (here mungbean). The analysis of all manure-amended plots (farmyard and green manure combined) showed that organic manure could replace approximately 50% of the mineral N recommended for sandy soils. In addition, the use of organic amendments entailed significant increases in residual soil C and N after only one cropping cycle, suggesting that soil fertility may be enhanced in the long-term. In the case of mungbean green manure, the N2 fixation measured by the δ15N natural abundance varied from 9 to 78 kg N ha-1 (average of 36 kg N ha-1). Highest N2 fixation was associated with low rainfall intensity during legume establishment and the absence of soil flooding during the pre-rice period. Only on deep sandy soils, the addition of 10 kg P ha-1 to mungbean was able to more than double the amount of N2 fixation compared to the legume without P amendment. The incorporation of P-amended legume residues (total biomass or after grain harvest) produced in both soil types rice grain yields that were comparable those obtained with the recommended mineral N application rate. Similarly, farmyard manure applied at 60 kg N ha-1 produced a rice yield comparable to mineral fertilizer N, however only in the deep soils. The study of farmers’ perception and adoption of organic amendments highlights that the use of farmyard manure is widespread but that its efficiency to replace mineral N is highly soil-specific. However, the availability of farmyard manure will be increasingly constrained by declining cattle numbers. While the adoption of legume green manures is potentially high, their actual use is constrained by soil P availability and limited to sites without soil flooding during the pre-rice niche and to systems with sufficient labor availability of biomass incorporation. While organic amendments have the potential to replace mineral fertilizers, such options and use strategies are highly site- and system specific.},
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