Effects of manure with different C/N ratios on yields, yield components and matter balances of organically grown vegetables on a sandy soil of northern Oman.

摘要

Little is known about how organic matter quality affects crop production and matter fluxes in irrigated organic agriculture under subtropical conditions. To contribute to filling this knowledge gap, river buffalos were fed with two diets characterised by a carbonitrogen (C/N) ratio of 11.0 and 27.2 and a ratio of neutral detergent fibre (NDF; i.e. cellulose, hemicellulose, lignin)/soluble carbohydrates (SC; i.e. sugars and starch) of 2.1 and 16.1, respectively. The diets yielded two manure types with a C/N ratio of 19 and a fibre/soluble carbohydrate (NDF/SC) ratio of 17 (ORG1) and 25 and 108 (ORG2), respectively. These uncomposted manures were split-applied at respective rates of 16 and 12 t ha-1 (ORG1) and 22 and 16 t ha-1 (ORG2) for 2 years in a factorial cropping systems experiment consisting of a cropping sequence of radish followed by either cauliflower (radish-cauliflower) or carrot (radish-carrot). The control treatment consisted of an equivalent combination of mineral fertilizers (MIN). Target levels of nitrogen (N), phosphorus (P) and potassium (K) were 590 kg, 251 kg and 320 kg ha-1, respectively. Despite consistently higher radish yields in ORG1 compared to ORG2 plots, these differences were not significant. Cauliflower yield, stem diameter and plant height (P < 0.001) increased with higher NPK availability from ORG2 to ORG1 and MIN, whereas carrot root length increased from ORG1 to MIN to ORG2 treatments (P < 0.001). Estimated N and P balances were positive across soil amendments (361 kg N ha-1 and 196 kg P ha-1 for radish-carrot and 299 kg N ha-1 and 184 kg P ha-1 for radish-cauliflower), but K balances were negative (-59 and -73 kg ha-1). Estimated C balances were strongly negative on MIN plots for both cropping systems (-7.3 and -5.0 t C ha-1). The results underline the difficulty of maintaining soil organic C under irrigated subtropical conditions with year-round high ambient temperatures.