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A long-term experiment (38 years) was conducted to elucidate the effects of long-term N addition on the net N mineralization in both paddy and upland soils, based on their initial soil N status, with and in connection with soil microbial biomass and N cycling extracellular enzyme activities. Two treatments without N addition CK (No fertilizer) and K (inorganic potassium fertilizer) and two treatments with N addition N (inorganic nitrogen fertilizer) and NK (inorganic nitrogen and potassium fertilizer) were placed in incubation for 90 days. Results showed that the total N and soil organic carbon (SOC) contents were higher in the treatments with N application compared to the treatments without N in both paddy and upland soils. The SOC content of paddy soil was increased relative to upland soil by 56.2%, 45.7%, 61.1% and 62.2% without N (CK, K) and with N (N and NK) treatments, respectively. Site-wise, total N concentration in paddy soil was higher by 0.06, 0.10, 0.57 and 0.60 times under the CK, K, N and NK treatments, respectively, compared with upland soil. In paddy soil, soil microbial biomass nitrogen (SMBN) was higher by 39.6%, 2.77%, 29.5% and 31.4%, and microbial biomass carbon (SMBC) was higher by 11.8%, 11.9%, 10.1% and 12.3%, respectively, in CK, K, N and NK treatment, compared with upland soil. Overall, compared to upland soil, the activities of leucine-aminopeptidase (LAP) were increased by 31%, 18%, 20% and 11%, and those of N-acetyl-b-D-glucosaminidase (NAG) were increased by 70%, 21%, 13% and 18% by CK, K, N and NK treatments, respectively, in paddy soil. A significantly linear increase was found in the NO3−-N and NH4+-N concentrations during the 90 days of the incubation period in both soils. NK treatment showed the highest N mineralization potential (No) along with mineralization rate constant, k (NMR) at the end of the incubation. SMBC, SMBN, enzyme activities, NO3−-N and NH4+-N concentrations and the No showed a highly significant (p ≤ 0.05) positive correlation. We concluded that long-term N addition accelerated the net mineralization by increasing soil microbial activities under both soils.

Fertilization regimes greatly influence how manganese (Mn) oxides are recycled. However, information on this subject is scarce in rice managed paddies. The objective of this study was to examine the distributions of Mn pools in bulk soil and aggregates under different chemical and organic fertilizer ratios at both surface (0-20 cm) and subsurface (20-40 cm) soil depths. Depth distributions of Mn pools in bulk soil and aggregates were extracted using Mehlich 3 (M3-Mn), dithionite (Mnd), oxalate (Mno), and pyrophosphate (Mnp) solutions under a long-term fertilization experiment in a double rice cropping system. Fertilizer treatments comprised unfertilized control (CK), mineral nitrogen, phosphorus, and potassium (NPK), 30% mineral NPK + 70% organic fertilizer (30F; pig manure + milk vetch), 50% mineral NPK + 50% organic fertilizer (50F), and 70% mineral NPK + 30% organic fertilizer (70F) under a randomly complete block design (RCBD) in paddy conditions. A pattern of 30F > 50F > 70F > NPK > CK was established with positive and significant correlations among total organic carbon (TOC), total nitrogen (TN), and pH. All pools of Mn were in higher concentrations at the subsurface compared to the surface layer, where all aggregate sizes contributed equally to the enrichment of the metal oxides except for Mno in the 0.25-2.0 mm sizes at the surface layer. Distribution of manganese oxides was equal among the different soil aggregates which could cause the microaggregates to transport Mn oxides and pollute both surface and underground water sources.

Background One main anthropogenic activity through which aggregate stability and elements are impacted in soils is through fertilization. Inputs of phosphate fertilizers to soils, for instance, have direct impacts on other elements in soils. Organic fertilizer sources are also known to markedly influence the availability of microelements. However, the extent to which different combination ratios of organic and chemical fertilizers affect the availability of these microelements in paddy soil aggregates is not clear. Aim Therefore, this study examined aggregate stability and the distribution pattern of various extractable aluminum, and iron oxides as influenced by a long‐term varied combinations of pig manure with chemical nitrogen, phosphorus, and potassium (NPK) fertilizer in a rice‐managed paddy. Method Mehlich 3, dithionite, oxalate, and pyrophosphate solutions were used to extract oxides of Al and Fe in bulk soil, and aggregates under long‐term fertilizer combinations, which included unfertilized CK (control), chemical NPK, 30% chemical NPK + 70% pig manure, 50% chemical NPK + 50% pig manure, and 70% chemical NPK + 30% pig manure in a randomly complete block design. Results The highest manure treatment (70% manure) markedly increased total organic carbon (TOC), and total nitrogen (TN) in contrast to other treatments. Soil pH was higher in all manure‐treated fertilizers than in chemical NPK fertilizer likely because organic matter (OM) neutralized the protons. Additionally, the highest manure treatment led to the creation of more macroaggregates (2–0.25 mm), which translated into higher aggregate stability indices such as mean weight diameter (MWD), and geometric mean diameter (GMD) compared to other treatments. It was also noted that the 70% manure treatment in comparison with other treatments, was associated with higher concentrations of extractable Al and Fe oxides. It was, however, not clear if the changes in extractable metals detected in this treatment resulted from OM‐driven redistribution of metals or if the manure contained some Al and Fe itself. Conclusion The 70% manure treatment increased TOC, TN, pH, and aggregate stability indices (higher MWD and GMD). This fertilizer treatment was also associated with more extractable Al and Fe oxides. For more sustainable rice‐managed paddies, the 70% manure treatment should be considered over the other fertilizer combinations.

Legumes, in symbiotic association with Rhizobia, are able to fix atmospheric N. Six local lima bean (Phaseolus lunatus) cultivars were grown under rainfed conditions in a coastal savannah environment. Objectives of the study were to evaluate the nodulation and fixed atmospheric N levels of the six local lima bean cultivars using both the 15 N isotope dilution method and N difference method (NDM). The linear relationship between fixed atmospheric N estimated using the 15 N isotope dilution method and NDM, was also assessed. The experiment was arranged in a randomized complete block design (RCBD) in three replicates with seven treatments, comprising six lima bean cultivars (B1, B2, B3, B4, B5 and B6) and the early maturing local maize variety, "Doke", as the reference crop. Total, effective nodules (EN) and non-effective nodules (NEN) were determined on 42 and 56 days after planting (DAP). The 15 N isotopic dilution method and NDM were used to quantify the fixed atmospheric N by the lima bean cultivars on 60 DAP. Effective root nodules per plant (EN) on 56 DAP ranged from 0.71 to 1.22, with the lima bean cultivar B4 having the highest value and cultivars B2 and B5 having the lowest value of EN, respectively. Similarly on 56 DAP, the lima bean cultivar B4 had the highest NEN value while cultivars B1, B2 and B5 had the lowest NEN value of 0.71 per plant. The mean fixed atmospheric N was 8.98 kg•ha −1 , based on the 15 N isotope dilution method, which was lower than 10.13 kg•ha −1 of fixed atmospheric N determined using NDM. The linear relationship between fixed atmospheric N estimated using the 15 N isotope dilution method and that estimated using the NDM, was positive but of average quality as the R 2 value was 0.56. Consequently, the linear model obtained from this relationship is moderate as 56% of the data used for the linear regression analysis were accounted for by the linear regression model developed. However, NDM could be used for fast screening to select lima bean cultivars for a more D. K. Asare et al. 926 detailed study to identify cultivars with promising fixed atmospheric N capabilities. Generally, results of the study provide opportunities for designing breeding and other agronomic programmes for enhancing the productivity and N-fixing capacity of local lima beans in the coastal savannah environment.