The Role of Organic Carbon in the Mineralization of Nitrogen, Carbon and Some of Nutrient Concentrations in Soil Salinity Conditions

The salt-affected soils are widely distributed in arid and semi-arid areas including Iran. In the soils affected by excessive salt, physical, chemical and biological changes in coupling with the low levels of organic matter (OM) resulting from the weak growth of plants, cause the deficiency of such nutrients as nitrogen (N) on the one hand and ionic toxicity (sodium and chlorine) on the other hand. The lack of vegetation in arid and saline areas results in the return of small amount of plant residues, so that the content of soil organic matter and thus the amount of nitrogen and other elements is reduced. One of the ways to increase the vitality and efficiency of elements in saline conditions for plants is to use organic fertilizers. Therefore, the aim of this study was to investigate the effects of organic carbon and salinity levels on nitrogen and carbon mineralization and nutrient concentrations in soil.

An completely randomized design (CRD) experiment was conducted to investigate the effect of organic carbon levels from the source of cow manure on the mineralization of carbon and nitrogen and nutrient concentrations in soil salinity conditions in a completely randomized design (CRD) by factorial arrangement with three replications. Organic carbon factor included (0, 1.5 and 3% organic carbon) and salinity factor consisted of 1.5, 4.5 and 9 dS/m. To create salinity levels a combination of MgSO4.7H2O, NaCl, Na2SO4 and CaCl2 was used in the ratio of 42.82, 0.91, 36.20, and 36.91, respectively. The treated soils were incubated at 25 °C under 70% field capacity for 70-day period. To determine the nitrification rate (Rn), the ammonium and nitrate concentrations were monitored during the incubation period at 0, 2, 4, 6, 8, and 10 weeks since the time of incubation. To address the carbon mineralization rate, the soil basal respiration was determined weekly since the beginning of the experiment. At the end of the experiment, the macro and micro nutrient status was determined in the treated soils.

The results showed that both salinity and organic matter application significantly affected the basal respiration, concentration of ammonium, nitrate, K, Ca, Mg, Na, Cl, Fe (P<0.01). Salinity increased soil Ca and Mg concentration. Organic carbon treatments have a significant effect on soil total nitrogen and soil organic carbon. The interactional effect of organic matter and salinity was significant on the basal respiration concentration of ammonium and nitrate and K, Na, Cl, Fe. Salinity in 9 dS.m-1 level compare to 1.5 dS.m-1 decreased basal respiration, NH4+ and NO3- concentration by 47%, 27% and 76% respectively. The basal respiration in 3% of soil organic carbon treatment, higher by 24% compared to control treatment (without organic matter addition). Furthermore, the ammonium and nitrate production after 70 days in 3% of soil organic carbon treatment, was 10% and 37% more than control.

Salinity had a negative effect on the process of carbon and nitrogen mineralization. The use of organic compounds, by creating a balance in nutrient status, could create more favorable conditions for the processes of mineralization of carbon and nitrogen. Organic compounds may have an easy source of carbon, which, if placed on the soil, stimulate and increase microbial activity, increase the mineralization of carbon and nitrogen to some extent.