Nutrient and carbon balances in organic vegetable production on an irrigated, sandy soil in northern Oman

Abstract

Our understanding of nutrient and carbon (C) fluxes in irrigated organic cropping systems of subtropical regions is limited. Therefore, leaching of mineral nitrogen (N) and phosphorus (P), gaseous emissions of NH 3 , N 2 O, CO 2 , and CH 4 , and total matter balances were measured over 24 months comprising a total cropping period of 260 d in an organic-cropping-systems experiment near Sohar (Oman). The experiment on an irrigated sandy soil with four replications comprised two manure types (ORG1 and ORG2) characterized by respective C : N ratios of 19 and 25 and neutral detergent fiber (NDF)-to-soluble carbohydrates (SC) ratios of 17 and 108. A mineral-fertilizer (MIN) treatment with equivalent levels of mineral N, P, and potassium (K) served as a control. The three treatments were factorially combined with a cropping sequence comprising radish (Raphanus sativus L.) followed by cauliflower (Brassica oleracea L. var. botrytis) or carrot (Daucus carota subsp. sativus). Over the 24-months experimental period gaseous N emissions averaged 45 kg ha -1 (59% NH 3 -N, 41%N 2 O-N) for MIN, 55 kg N ha -1 (69% NH 3 -N, 31%N 2 O-N) for ORG1, and 49 kg N ha -1 (59% NH 3 -N, 41% N 2 O-N) for ORG2. Carbon losses were 6.2 t ha -1 (98% CO 2 -C, 2% CH 4 -C) for MIN, 9.7 t C ha -1 (99% CO 2 -C, 1% CH 4 -C) for ORG1, and 10.6 t ha -1 (98% CO 2 -C, 2% CH 4 -C) for ORG2. Exchange resin-based cumulative leaching of mineral N amounted to 30 kg ha -1 for MIN, 10 kg ha -1 for ORG1, and 56 kg ha -1 for ORG2. Apparent surpluses of 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 were accompanied by K deficits of -59 kg ha -1 and -73 kg ha -1 , respectively, for both cropping systems. Net C balances for MIN, ORG1, and ORG2 plots were -7.3, -3.1, and 1.5 t C ha -1 for radish-carrot and -5.0, 1.3, and 4.6 t C ha -1 for radish-cauliflower. The results underline the difficulty to maintain soil C levels in intensively cultivated, irrigated subtropical soils.

Carbon and nutrient balances in three mountain oases in Northern Oman

“…This it is likely due to the very high temperatures in the Omani lowlands. Regardless of the cropping system, cumulative annual leaching losses of mineral N and P were very low compared to the findings from sandy lowland soils by Siegfried et al (2011). These low rates may be due to a combination of low seepage, differences in the particle size distribution and the higher organic C content of our soils.…”

“…2). In their study of gaseous N and C losses from the northern Oman costal plain of Al-Batinah, Siegfried et al (2011) reported similarly high fluxes that were related to a very fast C and nutrient turnover. Although they used lower manure application rates than in our study, total gaseous N and C losses were similar.…”

“…To this end PVC-cartridges were filled with a 2:3 mixture of anion-cation exchange resins and pure silica sand of 120-700 µm (Majan Glass Co., Sohar, Oman; Siegfried et al, 2011). For each cropping system, seven cartridges were buried in each of the selected fields below the rooting zone at 0.50 m depth and removed after each crop harvest or annually for pomegranate and date palm.…”

“…Calculations of annual cumulative leaching losses of N and P were made following the approach described by Siegfried et al (2011).…”

Advances and Trends in Organic Fruit and Vegetable Farming Research

“…For example, in an organic vegetable cropping system, compost N mineralization was not synchronous with N assimilation, resulting in a leaching risk under inappropriate practices such as the absence of a winter cover crop (Evanylo et al 2008). Furthermore, for irrigated organic vegetables grown under arid subtropical climate conditions, a very fast turnover of the amendments resulted in high C and N volatilization (Siegfried et al 2011). Consequently, sources, rate, timing, and method of amendment application will influence soil temperature, soil moisture, and nutrient availability (Gaskell and Smith 2007).…”