Clayton R. Butterly

Rhizosphere priming effect on soil organic carbon decomposition under plant species differing in soil acidification and root exudation.

Effects of rhizosphere properties on the rhizosphere priming effect (RPE) are unknown. This study aimed to link species variation in RPE with plant traits and rhizosphere properties. Four C3 species (chickpea, Cicer arietinum; field pea, Pisum sativum; wheat, Triticum aestivum; and white lupin, Lupinus albus) differing in soil acidification and root exudation, were grown in a C4 soil. The CO2 released from soil was trapped using a newly developed NaOH-trapping system. White lupin and wheat showed greater positive RPEs, in contrast to the negative RPE produced by chickpea. The greatest RPE of white lupin was in line with its capacity to release root exudates, whereas the negative RPE of chickpea was attributed to its great ability to acidify rhizosphere soil. The enhanced RPE of field pea at maturity might result from high nitrogen deposition and release of structural root carbon components following root senescence. Root biomass and length played a minor role in the species variation in RPE. Rhizosphere acidification was shown to be an important factor affecting the magnitude and direction of RPE. Future studies on RPE modelling and mechanistic understanding of the processes that regulate RPE should consider the effect of rhizosphere pH.

Free-air CO2 enrichment (FACE) reduces the inhibitory effect of soil nitrate on N2 fixation of Pisum sativum

BACKGROUND AND AIMS Additional carbohydrate supply resulting from enhanced photosynthesis under predicted future elevated CO2 is likely to increase symbiotic nitrogen (N) fixation in legumes. This study examined the interactive effects of atmospheric CO2 and nitrate (NO3(-)) concentration on the growth, nodulation and N fixation of field pea (Pisum sativum) in a semi-arid cropping system. METHODS Field pea was grown for 15 weeks in a Vertosol containing 5, 25, 50 or 90 mg NO3(-)-N kg(-1) under either ambient CO2 (aCO2; 390 ppm) or elevated CO2 (eCO2; 550 ppm) using free-air CO2 enrichment (SoilFACE). KEY RESULTS Under aCO2, field pea biomass was significantly lower at 5 mg NO3(-)-N kg(-1) than at 90 mg NO3(-)-N kg(-1) soil. However, increasing the soil N level significantly reduced nodulation of lateral roots but not the primary root, and nodules were significantly smaller, with 85% less nodule mass in the 90 NO3(-)-N kg(-1) than in the 5 mg NO3(-)-N kg(-1) treatment, highlighting the inhibitory effects of NO3(-). Field pea grown under eCO2 had greater biomass (approx. 30%) than those grown under aCO2, and was not affected by N level. Overall, the inhibitory effects of NO3(-) on nodulation and nodule mass appeared to be reduced under eCO2 compared with aCO2, although the effects of CO2 on root growth were not significant. CONCLUSIONS Elevated CO2 alleviated the inhibitory effect of soil NO3(-) on nodulation and N2 fixation and is likely to lead to greater total N content of field pea growing under future elevated CO2 environments.

Long-term stabilization of crop residues and soil organic carbon affected by residue quality and initial soil pH

Residues differing in quality and carbon (C) chemistry are presumed to contribute differently to soil pH change and long-term soil organic carbon (SOC) pools. This study examined the liming effect of different crop residues (canola, chickpea and wheat) down the soil profile (0-30cm) in two sandy soils differing in initial pH as well as the long-term stability of SOC at the amended layer (0-10cm) using mid-infrared (MIR) and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. A field column experiment was conducted for 48months. Chickpea- and canola-residue amendments increased soil pH at 0-10cm in the Podzol by up to 0.47 and 0.36units, and in the Cambisol by 0.31 and 0.18units, respectively, at 48months when compared with the non-residue-amended control. The decomposition of crop residues was greatly retarded in the Podzol with lower initial soil pH during the first 9months. The MIR-predicted particulate organic C (POC) acted as the major C sink for residue-derived C in the Podzol. In contrast, depletion of POC and recovery of residue C in MIR-predicted humic organic C (HOC) were detected in the Cambisol within 3months. Residue types showed little impact on total SOC and its chemical composition in the Cambisol at 48months, in contrast to the Podzol. The final HOC and resistant organic C (ROC) pools in the Podzol amended with canola and chickpea residues were about 25% lower than the control. This apparent priming effect might be related to the greater liming effect of these two residues in the Podzol.

Carbon Compounds Differ in Their Effects on Soil pH and Microbial Respiration

The mechanisms of soil pH change after the addition of organic matter to soil are not fully understood. The aim of this study was to investigate changes in pH after addition of carbon compounds over a 60-d incubation period. Seven organic compounds commonly found in plant residues (acetic acid, malic acid, citric acid, benzoic acid, ferulic acid, glucosamine hydrochloride and glucose) were selected based on the number and type of functional groups, and added at 0.5 mg C·g−1 soil to two soils differing in initial pH. Addition of organic acids (R-COOH) immediately decreased pH. The magnitude of the pH decrease depended on dissociation constant of the acid and the initial soil pH. In subsequent incubation, pH was slowly returned to original levels as organic anions were mineralized, consuming H+ ions. Glucose which contains hydroxyl (R-OH) group did not alter soil pH. However, carboxyl (R-COOH) and amine (R-NH2) groups changed pH significantly. Soil respiration was also increased by the addition of C compounds. Cumulative respiration was higher in soil with malic acid, citric acid, ferulic acid and glucose than with other compounds. The addition of glucose, citric acid and malic acid resulted in priming as cumulative respiration was greater than the actual amount of C added.

Is the Alkalinity within Agricultural Residues Soluble

A laboratory experiment was carried out to determine the contribution of whole residues of canola, chickpea and wheat and their fractions (insoluble/soluble) to soil pH changes during a 14-day incubation. Residues were added (1% w/w) to Frankston and Shepparton soils of initial pH of 4.45 and 6.20, respectively. Increases in pH were greatest for chickpea, less for canola and the least for wheat. The experiment confirmed that the soluble fraction of residues is important for the alkalinity release within initial stages of decomposition and also the source of components responsible for pH decreases in subsequent incubation. However, the relative differences of whole residues and the fractions were influenced by the initial soil pH.

Surface Amendments Can Ameliorate Subsoil Acidity in Tea Garden Soils of High-Rainfall Environments

Strongly acidic soils (pH < 5.0) are detrimental to tea (Camellia sinensis) production and quality. Little information exists on the ability of surface amendments to ameliorate subsoil acidity in the tea garden soils. A 120-d glasshouse column leaching experiment was conducted using commonly available soil ameliorants. Alkaline slag (AS) and organic residues, pig manure (PM) and rapeseed cake (RC) differing in ash alkalinity and C/N ratio were incorporated alone and in combination into the surface (0–15 cm) of soil columns (10 cm internal diameter × 50 cm long) packed with soil from the acidic soil layer (15–30 cm) of an Ultisol (initial pH = 4.4). During the 120-d experiment, the soil columns were watered (about 127 mm over 9 applications) according to the long-term mean annual rainfall (1 143 mm) and the leachates were collected and analyzed. At the end of the experiment, soil columns were partitioned into various depths and the chemical properties of soil were measured. The PM with a higher C/N ratio increased subsoil pH, whereas the RC with a lower C/N ratio decreased subsoil pH. However, combined amendments had a greater ability to reduce subsoil acidity than either of the amendments alone. The increases in pH of the subsoil were mainly ascribed to decreased base cation concentrations and the decomposition of organic anions present in dissolved organic carbon (DOC) and immobilization of nitrate that had been leached down from the amended layer. A significant (P < 0.05) correlation between alkalinity production (reduced exchangeable acidity – N-cycle alkalinity) and alkalinity balance (net alkalinity production – N-cycle alkalinity) was observed at the end of the experiment. Additionally, combined amendments significantly increased (P < 0.05) subsoil cation concentrations and decreased subsoil Al saturation (P < 0.05). Combined applications of AS with organic amendments to surface soils are effective in reducing subsoil acidity in high-rainfall areas. Further investigations under field conditions and over longer timeframes are needed to fully understand their practical effectiveness in ameliorating acidity of deeper soil layers under naturally occurring leaching regimes.

Alkalinity Generation by Agricultural Residues Under Field Conditions

The mechanisms of soil pH change by agricultural residues were investigated under field conditions. Residues of three important crop species, canola, chickpea and wheat, differing in alkalinity content and C to N ratio were incorporated into columns containing either Podosol (initial pH 4.5) or Tenosol (initial pH 6.2) soil. Net alkalinity production over the 27-month study depended on soil and residue type. Maximal alkalinity generated at 3 months by canola and chickpea residues was related to the alkalinity content (excess cation concentration) in the residues. Low initial pH reduced the rate and magnitude of alkalinity production. Amendment with wheat residue had little effect on alkalinity change. Net nitrification and nitrate leaching from 3 to 27 months reversed alkalinity generated during the initial period. However, a net increase in alkalinity was still observed in residue-amended soils 27 months after the initial application.

Soil Microbial Biomass and pH as Affected by the Addition of Plant Residues

The soil microbial biomass is involved in the decomposition of organic materials and thus, the cycling of nutrients in soils. Reductions in the size and activity of the microbial biomass are frequently used as an early indicator of changes in soil chemical and physical properties resulting from management and environmental stresses in agricultural ecosystems. In a laboratory-incubated soil, we found a strong relationship between microbial biomass C and microbial biomass N. Irrespective of the type of plant residues added, soil pH was significantly correlated with microbial biomass C and microbial biomass N. Different C/N ratio of the residues was the main characteristic that affected soil microbial biomass C, N and soil pH. Microbes played a main role in plant residues decomposition and indirectly influenced of soil pH.