Decreases in herbage production and of N uptake and utilization have been observed on Denbigh series soils in mid-Wales after several years in permanent pasture. Laboratory experiments were conducted to examine the contribution of denitrification to N loss from these wet grassland soils. Denitrification capacity was measured in seived soil following the addition of KNO 3 and maintained at 20°C under anoxic conditions. Emission of N 2 O was measured from intact field cores equilibrated under conditions of simulated "field capacity" using glucose as C substrate. The rate of loss of NO 3 − –N decreased with depth and in the 0–2.5 cm layer all added NO 3 − –N was lost in 10 d incubation. Net mineralization of NH 4 + –N occurred at about one-sixth of the rate of NO 3− –N disappearance. The presence of NO 3 − reduced the rate of decrease in redox potential (E h ) and the E h did not fall below about +200 mV until all NO 3 − –N had been lost. Emission of N 2 O was greatest between 6 and 48 h and denitrification rate decreased with depth. Addition of glucose increased N 2 O emission in the 2.5–5.7 cm layers indicating that C limitation to denitrification may occur at shallow depths in the soil profile of compacted grassland. On average, the total denitrification ranged between 15 and 20 kg N ha −1 , equivalent to 20–30% of applied N. The potential rates of denitrification change markedly over quite shallow depths in these compacted grassland soils. Furthermore, since denitrification occurred at substantial rates under simulated field capacity, conditions conducive to denitrification are likely to persist for quite long periods in the moist climatic conditions. Key words: Compacted soil, denitrification, glucose, grassland, nitrous oxide
Abstract The rate and timing of manure application when used as nitrogen (N) fertilizer depend on N‐releasing capacity (mineralization) of manures. A soil incubation study was undertaken to establish relative potential rates of mineralization of three organic manures to estimate the value of manure as N fertilizer. Surface soil samples of 0–15 cm were collected and amended with cattle manure (CM), sheep manure (SM), and poultry manure (PM) at a rate equivalent to 200 mg N kg−1 soil. Soil without any amendment was used as a check (control). Nitrogen‐release potential of organic manures was determined by measuring changes in total mineral N [ammonium‐N+nitrate‐N (NH4 +–N+NO3 −–N)], NH4 +–N, and accumulation of NO3 −–N periodically over 120 days. Results indicated that the control soil (without any amendment) released a maximum of 33 mg N kg−1soil at day 90, a fourfold increase (significant) over initial concentration, indicating that soil had substantial potential for mineralization. Soil with CM, SM, and PM released a maximum of 50, 40, and 52 mg N kg−1 soil, respectively. Addition of organic manures (i.e., CM, SM, and PM) increased net N released by 42, 25, and 43% over the control (average). No significant differences were observed among manures. Net mineralization of organic N was observed for all manures, and the net rates varied between 0.01 and 0.74 mg N kg−1 soil day−1. Net N released, as percent of organic N added, was 9, 10, and 8% for CM, SM, and PM. Four phases of mineralization were observed; initial rapid release phase in 10–20 days followed by slow phase in 30–40 days, a maximum mineralization in 55–90 days, and finally a declined phase in 120 days. Accumulation of NO3 −–N was 13.2, 10.6, and 14.6 mg kg−1 soil relative to 7.4 mg NO3 −–N kg−1 in the control soil, indicating that manures accumulated NO3 −–N almost double than the control. The proportion of total mineral N to NO3 −–N revealed that a total of 44–61% of mineral N is converted into NO3 −–N, indicating that nitrifiers were unable to completely oxidize the available NH4 +. The net rates of mineralization were highest during the initial 10–20 days, showing that application of manures 1–2 months before sowing generally practiced in the field may cause a substantial loss of mineralized N. The rates of mineralization and nitrification in the present study indicated that release of inorganic N from the organic pool of manures was very low; therefore, manures have a low N fertilizer effect in our conditions.
Phosphate solubilizing bacteria (PSB) play a significant role in plant P nutrition by their effect on soil P dynamics and their subsequent ability to make P available to plants via solubilization and mineralization processes. This study aimed to evaluate the effect of separate and combined use of indigenous PSB, poultry manure (PM) and compost on solubilization and mineralization of rock phosphate (RP) and their subsequent effect on growth and P accumulation of maize (Zea mays L.). A group of fifty seven bacteria were isolated from the rhizosphere/rhizoplane of maize that had been grown in soils collected from varying altitudes (655–2,576 m) of the mountain region of Rawalakot, Azad Jammu and Kashmir, Pakistan. After screening, the capacity of eleven isolates to solubilize mineral phosphate was quantitatively evaluated using insoluble Ca3(PO4)2 in culture medium as a time course study through spectrometer. The growth hormone producing (IAA) capacity of the isolates was also determined. Furthermore, five potential isolates were tested for their ability to increase P release capacity (mineralization) of insoluble RP in an incubation study. The effect of PSB inoculation on maize was determined in a completely randomized greenhouse experiment where root and shoot biomass and P accumulation in plants were assessed. The P solubilization index of selected isolates varied from 1.94 to 3.69, while the P solubilization efficiency ranged between 94.1% and 269.0%. The isolates MRS18 and MRS27 displayed the highest values. The P solubilization in the liquid medium was maximum at 6 and 9 days of incubation ranging between 9.91 and 44.04 µgmL−1 and the isolates MRS27 and MRS34 exhibited the highest solubilization. Six isolates showed additional capability of producing IAA ranging between 2.66 and 28.41 µgmL−1. Results of the incubation study indicated that P release capacity (P mineralization) of RP-amended soil varied between 6.0 and 11.8 µgPg−1 that had been significantly increased to 30.6–36.3 µgPg−1 (maximum value) when PSB were combined with RP. The combined application of PSB and organic amendments (PM, compost) with RP further increased P mineralization by releasing a maximum of 37.7 µgPg−1 compared with separate application of RP (11.8 µgPg−1) and organic amendments (21.5 and 16.5 µgPg−1). The overall effect of PSB (as a group) with RP over RP alone on maize growth showing a relative increase in shoot length 21%, shoot fresh weight 42%, shoot dry weight 24%, root length 11%, root fresh weight 59%, root dry weight 35% and chlorophyll content 32%. This study clearly indicates that use of PSB, and organic amendments with insoluble RP could be a promising management strategy to enhance P availability in soil pool and improve plant growth in intensive cropping systems.
The barometric process separation (BaPS) and 15 N dilution techniques were used to determine gross nitrification rates on the same soil cores from an old grassland soil. The BaPS‐technique separates the O 2 consumption into that from nitrification and that from soil organic matter (SOM) respiration. The most sensitive parameter for the calculations via the BaPS technique is the respiratory quotient (RQ = ΔCO 2 /ΔO 2 ) for SOM turnover (RQ SOM ). Combining both methods (BaPS– 15 N) allowed the determination of the RQ SOM The RQ value determined in such a way is adjusted for the influence of nitrification and denitrification, which are both characterized by totally different RQ values. The results for the grassland soil showed that 6 to 10% of O 2 was consumed by nitrification when incubated at 20°C and 0.49 g H 2 O g −1 soil. A set of BaPS measurements with the same soil at various temperature and moisture contents showed that up to 49% of the total O 2 consumption was due to nitrification. The calculated RQ SOM values via the BaPS– 15 N technique presented here are more closely associated with the overall SOM turnover than the usual net RQ reported in the literature. Furthermore, the RQ SOM value provides an overall indication of the decomposability and chemical characteristics of the respired organic material. Hence, it has the potential to serve as a single state index for SOM quality and therefore be a useful index for SOM turnover models based on substrate quality.
This study provided an insight on improving soil-plant micronutrients availability in response to poultry manure (PM), wheat milling residues (WMR) and urea N (UN) and their integration in wheat–soybean cropping system. The treatments were: control; poultry manure full, PM100; wheat milling residues full, WMR100; urea N full, UN100; PM half and WMR half, PM50+WMR50; UN50+PM50; UN50+WMR50; UN50+PM25+WMR25. All amendments were added at the rate or equivalent to 100 kg total N ha–1. Results indicated that the integrated treatments increased Cu, Fe, Mn and Zn uptake of wheat by 35.7–103%, 48.4–111.1%, 85.2–267.0% and 33.8–128.2%, respectively over control. In soybean the corresponding increase in micronutrient uptake (Cu, Fe, Mn and Zn) was 18.3–60.3%, 27.5–87.4%, 14.1–54.6% and 13.2–58.0% in integrated treatments. The post-harvest soil analysis indicated 2 to 3-fold increase in micronutrient content with highest values in PM100 i.e., 2.66 mg kg−1 for Cu, 14.41 mg kg−1 for Fe, 18.58 mg kg−1 for Mn and 2.44 mg kg−1 for Zn, respectively. The results showed that the PM either alone or in integrated with WMR and UN can be an effective management strategy for improving micronutrient content of soil–plant.
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