Nitrogen mineralization in sub‐tropical paddy soils in relation to soil mineralogy, management, pH, carbon, nitrogen and iron contents

Abstract

The nitrogen (N) requirement for paddy rice cultivated in Bangladesh amounts to approximately 80 kg N ha-1. Lack of knowledge on N mineralization from soil organic matter leads farmers to meet this N requirement exclusively by costly mineral fertilizers, which have typically an efficiency of less than 40%. We assessed to what extent routinely analysed soil properties (N and carbon (C), texture, pH, extractable iron (Fe), aluminium (Al) and manganese (Mn), soil mineralogy and length of the annual inundation period) are able to predict net aerobic and anaerobic N mineralization in paddy soils. Both soil N and C correlated positively with the aerobic but not with the anaerobic N mineralization rate. Instead, relative anaerobic N mineralization showed a significant negative correlation with soil N content. We observed no significant influence of clay mineralogy on soil N mineralization. Aerobic but not anaerobic N mineralization increased with length of the annual inundation period while the proportion of the soil N that was mineralized during 120 days decreased. The large clay content of fields that are inundated for 9–10 months annually explains the co-occurrence of large soil N contents and relatively small N mineralization rates in these fields. However, variation in texture did not explain variation in N mineralization of soils with inundation periods of 3–8 months. Instead, the anaerobic N mineralization correlated positively with Na pyrophosphate-extractable Fe and negatively with pH (both at P < 0.01). Thus, pH and Fe content, rather than soil N content, clay mineralogy or texture, explained the substantial variation in anaerobic N mineralization of paddy soils in Bangladesh inundated for 3–8 months. It is not known if these relationships between net evolution of ammonium in soil and pH and Fe content are causal or indirect. Elucidation of these mechanisms would greatly further our comprehension of the biochemistry of the young ‘floodplain soils' with relatively low content of pedogenic oxides throughout southeast Asia

Disponibilidade de nitrogênio para o arroz irrigado por inundação, pelo método de destilação a vapor direta

“…A limitação do índice MOS-rotina em estimar a disponibilidade de N para a cultura do arroz irrigado, observada no presente estudo, está de acordo com os resultados obtidos por Roberts et al (2012) e Kader et al (2013), que apontaram correlações insatisfatórias do C orgânico com o N mineralizável, com a produção de arroz e com a absorção de N pelas plantas. Roberts et al (2012) afirmam que, apesar da existência de relação intrínseca e indiscutível entre os teores de N total e C total, a relação entre essas variáveis e o potencial de mineralização em um determinado solo não está claramente definida.…”

Subcritical water extraction to isolate kinetically different soil nitrogen fractions

“…N mineralization was assessed earlier by Kader et al (2013) by monitoring the release of NH 4 and NO 3 under controlled aerobic and anaerobic conditions. Briefly, for each soil 42 PVC tubes (5 cm diameter) were filled with 200 g of dry soil, 21 (3 replicates × 7 dates) for aerobic and 21 (3 replicates × 7 dates) for anaerobic incubation.…”

“…The clay fraction's mineralogical composition was identified by X-ray diffraction on oriented samples after (i) K and Mg saturation, (ii) glycerol solvation, and (iii) drying of the K saturated samples at 300 and 550 • C for 2 h, as described in detail by Kader et al (2013).…”

“…A set of 25 paddy soils was previously collected (Kader et al, 2013) from agricultural fields throughout northern Bangladesh, which has a subtropical monsoon climate (annual mean temperature of 25.8 • C and annual precipitation of 2428 mm; Bangladesh Meteorological Department, 2011). Either one or two rice crops were cultivated per year on the sampled fields.…”

“…7438 S. Sleutel: Subcritical water extracted N from soils Table 1. Soil properties of sampled agricultural fields in Bangladesh (data from Kader et al, 2013 …”