Lime and/or Phosphate Application Affects the Stability of Soil Organic Carbon: Evidence from Changes in Quantity and Chemistry of the Soil Water-Extractable Organic Matter.

The mechanisms by which lime and/or phosphate addition impacts the preservation of soil organic matter (OM) are poorly understood. We explored the changes in quantity and chemistry of water-extractable organic matter (WEOM) in the bulk soil and its heavy density fraction (>1.6 g/cm3) of an unmanaged C-rich volcanic soil caused by lime and/or phosphate application. The addition of lime or phosphate caused (i) a significant increase in the WEOM, along with a decrease in its C/N ratio and an increase in its aromaticity, and (ii) changes in the WEOM chemical composition, measured with pyrolysis-gas chromatography/mass spectrometry, this being most impacted by lime application. The combined effect of lime and phosphate addition on the quantity and chemistry of WEOM was larger than the effects of separate lime and phosphate additions. By comparing the response of the bulk soil and the heavy fraction, we infer that phosphate has a greater contribution to the destabilization of vulnerable particulate OM, while lime causes a comparable disruption in the particulate OM and that in the heavy fraction. These findings provide a mechanistic insight into the decreased OM stability after liming and/or P fertilizing Andosols. They have implications for designing climate-smart management practices for these soils.

The regulators of soil organic carbon mineralization upon lime and/or phosphate addition vary with depth.

Knowledge of the key factors regulating soil organic carbon (OC) mineralization in response to fertilizers and lime application is essential to understanding the effects of agricultural land management on soil OC preservation. Microbial community composition and OC availability to microorganisms have been proposed as the two most imperative factors controlling soil OC mineralization, although their relative importance is still under debate. Here we performed a laboratory incubation in combination with high-throughput sequencing and structural equation modeling to examine the mechanisms underlying the responses of OC mineralization in the topsoil and the subsoil of a volcanic soil (an Andosol) to the additions of lime and/or phosphate. Results showed that lime and/or phosphate additions induced distinct shifts in the microbial community composition and functional profiles in the topsoil and the subsoil. We found that OC mineralization relied on microbial community composition and functionality in the topsoil but was strongly related to the quality and quantity of the water-extractable OC (indicative of the OC availability) in the subsoil. These data suggest that the key regulator controlling the response of OC mineralization to lime and/or P additions shifts from microbial community composition to OC availability as soil depth increases in the Andosol. Our findings highlight the central role of mechanisms controlling soil OC mineralization in regulating the responses of mineralization to intensive agricultural management practices.

Data on the organic matter characteristics of New Zealand soils under different land uses.

This article contains data related to the research article entitled "An Investigation of Organic Matter Quality and Quantity in Acid Soils as Influenced by Soil Type and Land Use" (Shen et al., 2018) [1]. The data was collected using a chemical fractionation scheme of soil organic matter (OM). This involved the separation of organic carbon (OC) fractions based on their solubility in (i) cold and hot water, (ii) 0.1 M sodium pyrophosphate (pH ~ 10), and (iii) 2% HF solution, and the residue remaining after the HF extraction. The OM in this residue, after treatment with 2% HF solution, was characterised using pyrolysis (Py)-GC/MS. This technique involves thermal decomposition of OM into various pyrolysis products, which are then chromatographically separated and determined by mass spectroscopy. This technique has been used to semi-quantify individual soil OM constituents so that in-depth information on soil OM molecular fingerprints is provided. This article presents a detailed dataset of physical-chemical characterization, OC fractions and OM molecular fingerprints of 62 soil samples for a range of soil orders (i.e., Allophanic, Brown, Gley, Pallic and Recent) and land uses (i.e., permanently grazed pasture, ungrazed/unmanaged grasslands, annual cropping) across New Zealand. Principal component analysis was used to investigate the relationships of different soil properties with OC fractions and OM chemistry so that the underlying mechanisms responsible for the differences encountered in OM quantity and quality between soil orders and land uses are understood.

Testing an Alternative Method for Estimating the Length of Fungal Hyphae Using Photomicrography and Image Processing.

This study aimed to develop and test an unbiased and rapid methodology to estimate the length of external arbuscular mycorrhizal fungal (AMF) hyphae in soil. The traditional visual gridline intersection (VGI) method, which consists in a direct visual examination of the intersections of hyphae with gridlines on a microscope eyepiece after aqueous extraction, membrane-filtration, and staining (e.g., with trypan blue), was refined. For this, (i) images of the stained hyphae were taken by using a digital photomicrography technique to avoid the use of the microscope and the method was referred to as "digital gridline intersection" (DGI) method; and (ii), the images taken in (i) were processed and the hyphal length was measured by using ImageJ software, referred to as the "photomicrography-ImageJ processing" (PIP) method. The DGI and PIP methods were tested using known grade lengths of possum fur. Then they were applied to measure the hyphal lengths in soils with contrasting phosphorus (P) fertility status. Linear regressions were obtained between the known lengths (Lknown) of possum fur and the values determined by using either the DGI (LDGI) (LDGI = 0.37 + 0.97 × Lknown, r2 = 0.86) or PIP (LPIP) methods (LPIP = 0.33 + 1.01 × Lknown, r2 = 0.98). There were no significant (P > 0.05) differences between the LDGI and LPIP values. While both methods provided accurate estimation (slope of regression being 1.0), the PIP method was more precise, as reflected by a higher value of r2 and lower coefficients of variation. The average hyphal lengths (6.5-19.4 m g-1) obtained by the use of these methods were in the range of those typically reported in the literature (3-30 m g-1). Roots growing in P-deficient soil developed 2.5 times as many hyphae as roots growing in P-rich soil (17.4 vs 7.2 m g-1). These tests confirmed that the use of digital photomicrography in conjunction with either the grid-line intersection principle or image processing is a suitable method for the measurement of AMF hyphal lengths in soils for comparative investigations.

Biochar in co-contaminated soil manipulates arsenic solubility and microbiological community structure, and promotes organochlorine degradation.

We examined the effect of biochar on the water-soluble arsenic (As) concentration and the extent of organochlorine degradation in a co-contaminated historic sheep-dip soil during a 180-d glasshouse incubation experiment. Soil microbial activity, bacterial community and structure diversity were also investigated. Biochar made from willow feedstock (Salix sp) was pyrolysed at 350 or 550°C and added to soil at rates of 10 g kg-1 and 20 g kg-1 (representing 30 t ha-1 and 60 t ha-1). The isomers of hexachlorocyclohexane (HCH) alpha-HCH and gamma-HCH (lindane), underwent 10-fold and 4-fold reductions in concentration as a function of biochar treatment. Biochar also resulted in a significant reduction in soil DDT levels (P < 0.01), and increased the DDE:DDT ratio. Soil microbial activity was significantly increased (P < 0.01) under all biochar treatments after 60 days of treatment compared to the control. 16S amplicon sequencing revealed that biochar-amended soil contained more members of the Chryseobacterium, Flavobacterium, Dyadobacter and Pseudomonadaceae which are known bioremediators of hydrocarbons. We hypothesise that a recorded short-term reduction in the soluble As concentration due to biochar amendment allowed native soil microbial communities to overcome As-related stress. We propose that increased microbiological activity (dehydrogenase activity) due to biochar amendment was responsible for enhanced degradation of organochlorines in the soil. Biochar therefore partially overcame the co-contaminant effect of As, allowing for enhanced natural attenuation of organochlorines in soil.