Effects of Spent Mushroom Substrates and Alfalfa Green Manure on Selected Fertility Indicators of Soil Quality and Spinach’s Nutrients

IntroductionConventional cropping systems, dependent on heavy application of chemical fertilizers, are not ecologically and environmentally sustainable; they are a threat for soil and water quality and, in consequence, for plant and human health. Nitrogen fertilizers are heavily applied in conventional leaf vegetable production systems to obtain maximum growth and yield. However, the excess nitrogen tends to accumulate in leaf vegetables in the form of nitrate, which pose serious human health hazards. Therefore, to supply nitrogen from non-chemical sources, such as organic amendments, is a sustainable practice for production of leaf vegetables. Spent mushroom substrate (SMS), which is the remaining material after the harvest of mushroom, is produced in large quantities (5 kg SMS for 1 kg of mushroom) and is enriched with organic carbon, N, P, K, and micronutrients. Therefore its reuse as a soil amendment not only provides essential elements for plants but also improves soil quality. Similarly, incorporation of green manures, especially legume green manures, into cropping systems is a sustainable practice for soil fertility and soil quality management. In this study, we aimed to investigate the short-term effects of two soil organic amendments (spent mushroom substrate and alfalfa residues) and their combination, in comparison to inorganic N fertilizer (urea), on soil fertility, and selected essential nutrients, and nitrate accumulation in a leaf vegetable, test plant (spinach).    Materials and MethodA one-season pot experiment was led in a randomized complete block design with three replications in experimental greenhouse of Bu-Ali Sina University. Treatments were comprised of two levels of spent mushroom substrate (SMS-1: 2% SMS, and SMS-2: 5% SMS), two levels of alfalfa green manure (AGM-1: 1% AGM, and AGM-2: 3% AGM); two levels of the mixture of SMS and AGM (SMS+AGM-1: 1% SMS plus 0.5% AGM; and SMS+AGM-2: 2.5% SMS plus 1.5% AGM);  two levels of urea fertilizer (U-1; 120 kg/ha, and U-2: 360 kg/ ha); and control. Selected properties of the initial soil and both organic amendments were determined. Spinach (Spinacea oleracea L.) was seeded as leaf vegetable, test plant in early autumn 2017. After ten weeks, spinach were harvested and the aboveground and root dry weights were determined. Moreover, the content of NO3-, P, Fe, Cu, Zn, and Mn in edible parts were measured. Soil samples were analyzed for EC, pH, total organic carbon, available P and K, and alkaline phosphatase activity.Results and DiscussionAll soil quality indicators were significantly affected by the treatments. TOC was significantly increased in all of the organic treatments compared to the chemical and control treatments. The maximum increase in TOC was observed in SMS-2, SMS+AGM-2, and AGM-2 treatments, compared to the control (134, 130 and 107%, respectively). A decreasing trend in TOC was detected in the high level of urea treatment (U-2) compared to the control which can be explained by the faster decomposition of soil organic matter in the presence of higher inorganic N inputs. Both organic amendments (in both levels) and the higher level of urea (U-2) decreased soil pH compared to the control. The initial low pH of SMS (5.6) and AGM (6.2), in the first case, and oxidation of urea to nitrate, in the latter, may justify this observation. In contrast, soil EC increased under the both organic amendments relative to the control and U-1 treatments. Moreover, the adverse effect of SMS on soil salinity was greater than AGM due to the initial differences in their corresponding source materials (5.8 vs. 3.0 ds/m). Available K was significantly increased in the second level of all organic treatments compared to the chemical and control treatments. As for available P, all organic treatments, except AGM-1, led to the significantly higher P than the chemical and control treatments. It is reported that organic materials compete with mineral particles for P adsorption and increase its availability. Moreover, all organic treatments, except SMS-1, significantly increased phosphatase activity compared to the chemical and control treatments. This could contribute to the mineralization of organic materials and increase available P.   Spinach yield was affected by the experimental treatments. The highest increase in shoot dry weight occurred in SMS+AGM-2 and AGM-2 treatments by 235 and 230%, respectively, compared to the control. Moreover, the second level of all organic treatments as well as the first level of SMS plus AGM treatment significantly increased yield compared to the chemical treatments. Spinach P content was significantly higher in all organic treatments, except SMS-1 and AGM-1, compared to the chemical and control treatments. Organic amendments, by decreasing the surface adsorption of P and increasing soil microbial biomass, promote the availability of P for plants. Spinach nitrate content ranged from 265 (in control) to 7807 mg/kg (in U-2). According to the critical limit of nitrate in spinach (4000 mg/kg) presented by European Union, only U-2 treatment led to over-accumulation of NO3-. The two levels of AGM treatments and SMS+AGM-2 resulted in the comparable amounts of nitrate as the recommended amount of urea (U-1). A narrow variation in spinach Cu content (from 6.1 in SMS+AGM-2 to 9.8 mg/kg in AGM-2), all within the standard range reported for plants (5-20 mg/kg), was observed among the treatments. Spinach Fe content was increased under all organic treatments relative to the control, although some disparities were not significant. The lowest Fe was detected in U-2. It is reported that excessive N may diminish root growth and, in turn, reduce nutrient uptake. Spinach Zn content varied from 44.8 (in control) to 71.5 mg/kg (in SMS-2), which was close to the higher limit of standard range (20-50 mg/kg) reported for vegetables, but lower than toxic concentration range (200-400 mg/kg). Spinach Mn content varied from 17.4 (in control) to 32.1 mg/kg (in SMS-2), which was close to the lower limit of the standard range (40-400 mg/kg) reported for plants.ConclusionThe most appropriate treatments in view of improving yield and soil quality (i.e., optimum TOC, P, and K; and lower EC) as well as tolerable nitrate accumulation were SMA+AGM-1 and SMS-1 in decreasing order. These treatments are preferred over the chemical treatments (U-1 and U-2).