Murray R. Davis

Seasonal changes in soil phosphorus and associated microbial properties under adjacent grassland and forest in New Zealand

Abstract Land-use change from grassland to short rotation plantation forest can have significant impacts on soil nutrient dynamics and microbial processes. Seasonal dynamics of soil phosphorus (P) and associated microbial properties were investigated in upper (0–5xa0cm) soils under adjacent unimproved grassland and a 19-year-old forest stand (mixture of Pinus ponderosa and Pinus nigra). Afforestation of grassland ameliorated upper soil moisture and temperature regimes, resulting in reduced but less variable soil moisture beneath forest, and reduced temperature extremes, with soil being cooler under forest in summer, but warmer in winter. Results from this study showed that levels of soil organic carbon (C), total nitrogen (N) and organic P fractions under grassland were consistently higher, but levels of inorganic P fractions (bicarbonate extractable Pi and total Pi), microbial biomass C and P, and phosphatase enzyme activities were lower compared with forest over all seasons. Similar seasonal patterns of soil P fractions under grassland and forest demonstrated that labile organic P was mineralized by increasing microbial activity to meet increasing plant demand in spring and summer, but accumulated as a result of increased organic inputs, slower plant growth and low microbial activity in late autumn and winter. It was concluded that P recycling was mainly driven by plant P demand and sustained by root litter inputs in the grassland ecosystem and leaf litter inputs in the forest ecosystem. Seasonal changes in environmental conditions (rainfall, soil moisture and temperature) influenced microbial processes involved in P cycling. Microbial biomass plays a pivotal role in P cycling. Annual release of P through microbial biomass was higher in the forest soil (16.1xa0kgxa0ha−1) than in the grassland soil (13.9xa0kgxa0ha−1). The turnover rate of biomass P was also higher in the forest soil (1.28 per year) than in the grassland soil (0.80 per year). In addition, abundant C and P (particularly labile forms) and high microbial and enzyme activities found in the forest floor highlight the importance of the forest floor in P cycling.

Decomposition and nutrient release from radiata pine (Pinus radiata) coarse woody debris

Abstract The dynamics of decomposition of thinning slash and nutrient release were studied in a radiata pine (Pinus radiata D Don) plantation forest in New Zealand. This study examined decomposition of coarse woody debris (CWD) components (log-wood, log-bark, and side branches) originating from stands thinned between 1 and 13 years previously. Changes in component density were used to estimate the decay rates. Both chemical analyses and 13 C nuclear magnetic resonance (NMR) spectroscopy were conducted to investigate relationships between decomposition and chemical composition. The rate of decomposition was the fastest for log-wood followed by log-bark, which in turn decomposed faster than side-branch material. After 13 years, log-wood, log-bark and side branches lost 59, 55 and 24% of their initial mass, respectively. Single exponential model analysis indicated that the half-life of total thinning slash (sum of log-wood, log-bark and side branches) was 13.25 years. Proximate analyses showed that the faster rate of decomposition of log-wood was mainly due to greater carbohydrate concentration, while greater concentrations of polyphenol and lignin were responsible for the slower decomposition rate of log-bark. The slow rate of decomposition of side branches was due to unfavorable micro-climate (most of the side branches were not in contact with soil even after 9 years of decomposition) as well as greater lignin and polyphenol concentrations. Carbon-13 NMR analysis revealed that during decomposition the relative proportions of O-alkyl and acetal C, which represent carbohydrates, decreased while N-alkyl, aromatic, and phenolic C, which represent tannins and acid insoluble compounds including lignin, increased in all thinning slash components. Net release of nutrients (N, P, K, Ca and Mg) occurred during thinning slash decomposition, in contrast to earlier studies, although the concentrations of most nutrients increased with time. Nutrient release was attributed to the nature of the thinning slash materials and the high proportion of bark material in particular. Although there was a net release, the rate of release of C and the majority of nutrients from thinning slash was slow making it an important C sink and long-term source of nutrients.

Changes in soil acidity and organic matter following the establishment of conifers on former grassland in New Zealand

Abstract Effects of a land use change from grassland to coniferous plantation forestry (Pseudotsuga menzieii [Douglas fir]; Pinus radiata [radiata pine]) on soil acidity and organic matter were assessed at two sites in New Zealand. The sites differed with respect to soils, climate, vegetation cover and type, relative maturity and management of the forest stands. Results obtained at the different sites were, therefore, not directly comparable, although they represented a comparison of a similar change in land use and some overall trends were evident. The change from grassland to conifers decreased levels of organic carbon, total nitrogen and exchangeable cations and increased exchangeable acidity in the upper 20–30xa0cm of soil. Exchangeable aluminium and exchangeable acidity were more sensitive measures of the effects of afforestation on soil acidity than pH.

Influence of conifers on the forms of phosphorus in selected New Zealand grassland soils

We examined the effects of conifers on the forms of P in low-fertility tussock grassland soils using 31P nuclear magnetic resonance (NMR) and soil P fractionation. Results from field and glasshouse experiments clearly demonstrated that conifers enhanced the mineralization of labile (and to a lesser extent more resistant) forms of soil organic P which, in turn, increased amounts of labile inorganic P in the soil. These findings have important implications for P availability and long-term sustainable management of grassland soils in New Zealand.

Decomposition and nutrient dynamics of green and freshly fallen radiata pine (Pinus radiata) needles

Thinning and pruning operations in radiata pine (Pinus radiata) plantation forests result in the addition of large amounts of green needles to the forest floor. The decomposition of green and freshly fallen radiata pine needles and the effects of adding green needles to freshly fallen needles were examined in a microcosm experiment. Green needles lost 72% of the original mass after 10 months, compared with 27% for freshly fallen needles. The corresponding mass losses for 1:1 ratios of green and freshly fallen needles were 55% when mixed and 53% when layered. Nutrient concentrations generally increased during decomposition while total amounts of nutrients decreased with time. Decomposition was primarily influenced by needle lignin and N content, and by the holocellulose to lignocellulose quotient (HLQ). The results of this study indicate that addition of green needles does not significantly affect the decomposition of freshly fallen needles. This outcome was attributed to substrate preference by decomposer microorganisms. It is, therefore, concluded that forest management practices (thinning, pruning and harvesting) which result in significant inputs of carbohydrates and nutrients in the form of green needles will have little impact on decomposition of existing forest floor materials.

Effects of plant species on microbial biomass phosphorus and phosphatase activity in a range of grassland soils

Soil P transformations are primarily mediated by plant root and soil microbial activity. A short-term (40 weeks) glasshouse experiment with 15 grassland soils collected from around New Zealand was conducted to examine the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil microbial properties and microbiological processes involved in P dynamics. Results showed that the effect of plant species on soil microbial parameters varied greatly with soil type. Concentrations of microbial biomass C and soil respiration were significantly greater in six out of 15 soils under radiata pine compared with ryegrass, while there were no significant effects of plant species on these parameters in the remaining soils. However, microbial biomass P (MBP) was significantly lower in six soils under radiata pine, while there were no significant effects of plant species on MBP in the remaining soils. The latter indicated that P was released from the microbial biomass in response to greater P demand by radiata pine. Levels of water soluble organic C were significantly greater in most soils under radiata pine, compared with ryegrass, which suggested that greater root exudation might have occurred under radiata pine. Activities of acid and alkaline phosphatase and phosphodiesterase were generally lower in most soils under radiata pine, compared with ryegrass. The findings of this study indicate that root exudation plays an important role in increased soil microbial activities, solubility of organic P and mineralization of organic P in soils under radiata pine.

Mineralisation of soil orthophosphate monoesters under pine seedlings and ryegrass

The effects of radiata pine (Pinus radiata D. Don) seedlings and ryegrass (Lolium perenne L.) on the mineralisation of orthophosphate monoesters in 7 grassland soils were assessed in a 10-month pot trial using NaOH–EDTA extraction and solution 31P NMR spectroscopy. Extraction with NaOH–EDTA recovered 46–86% of the total soil P, and NaOH–EDTA-extractable organic P determined by molybdate colourimetry ranged between 194 and 715 mg/kg soil, representing 34–85% of the total soil organic P. Orthophosphate monoesters were the predominant species of the extracted organic P in all soils, with much smaller concentrations of orthophosphate diesters, and traces of phosphonates. Concentrations of orthophosphate monoesters were consistently lower in soils under pine (103–480 mg P/kg soil) compared with the initial soils (142–598 mg P/kg soil) and most soils under grass (122–679 mg/kg soil). Mineralisation of myo-inositol hexakisphosphate accounted for 18–100% of the total mineralisation of orthophosphate monoesters in most soils under radiata pine. This suggests that supposedly recalcitrant inositol phosphates are available for uptake by radiata pine, although the extent of this varies among soils.

Characterization of phosphorus availability in selected New Zealand grassland soils

Appropriate evaluation of phosphorus (P) availability in soil is aprerequisite for ensuring the productivity and long-term sustainable managementof agroecosystems. Fifteen soils presently under grassland were collected fromdifferent areas of New Zealand and soil P availability was assessed by isotopicexchange kinetics (IEK) and related to P forms obtained by chemicalfractionation (sequential extraction). Concentrations of total P determined inthe 15 soils ranged from 375 to 2607 mg kg−1(mean1104 mg kg−1). Mean concentrations of inorganic P(Pi) extracted by sequential extraction with ammonium chloride, sodiumbicarbonate, sodium hydroxide (first), hydrochloric acid and sodium hydroxide(second) were 1.2, 41, 205, 113 and 23 mg kg−1,respectively. Mean concentrations of organic P (Po) extracted by sodiumbicarbonate, sodium hydroxide (first) and sodium hydroxide (second) were 133,417 and 105 mg kg−1, respectively. Similarly,results from IEK analysis showed that the intensity (water soluble Pi (Cp)),capacity (R/r1 and n), and quantity (E value,isotopically exchangeable P pools (E1 min,E1 min–24 h,E24 h–3 m,Eu2009>u20093 m)) factors varied markedlyamongst soils. Thus Cp concentrations ranged from 0.02–1.90 mgL−1, while concentrations of Pi determined in theE1 min, E1 min–24,E24 h–3 m,E>3 m pools were 2–29 (mean 10), 10–321(76), 11–745 (152), and 8–498 (177) mgkg−1, respectively. The corresponding values forR/r1 and n were 1.0–17.7 (mean 4.5) and0.10–0.50 (mean 0.37), respectively. Regression analysis revealed that Cpconcentrations were exponentially and inversely proportional toR/r1,n and P sorption index (PSI)(R2u2009=u20090.806(Pu2009<u20090.01), 0.852 (Pu2009<u20090.01) and 0.660(Pu2009<u20090.01), respectively). Cluster analysis identified twobroad groups of soils, namely those with low P availability (mean Cp0.11 mg L−1, E1 min Pi 5mg kg−1, R/r1 3.9,n 0.44), and those with high P availability (mean Cp 1.33mg L−1, E1 min Pi 20mg kg−1, R/r1 1.21,n 0.16). Correlation analysis indicated thatE1 min P i was significantly correlated with bicarbonateextractable Pi (BPi, R2u2009=u20090.37,Pu2009<u20090.05) and thesum of ammonium chloride extractable Pi (APi) and BPi(R2u2009=u20090.38,Pu2009<u20090.05). However, the concentration of Pi in theE1 min pool was generally lower than the sum of APi andBPi. Sodium hydroxide extractable Pi (N1Pi) was significantlycorrelated with the sum of the E1 min,E1 min–24 h,E24 h–3 m Pi pools(R2u2009=u20090.974, Pu2009<u20090.01),indicating that N1Pi fractioncould be considered as representing potentially available soil P for pasturespecies over a growing season.

Effects of land-use change from grassland to forest on soil sulfur and arylsulfatase activity in New Zealand

The effects of land-use change from grassland to forest on soil sulfur (S) and arylsulfatase enzyme activity were investigated by comparing soils under unimproved grassland and an adjacent 19-year-old exotic forest stand (mixture of Pinus ponderosa and P. nigra). Results showed that concentrations of organic S in topsoil under forest were significantly lower [418 µg/g (0–5 cm), 398 µg/g (5–10 cm)] than corresponding soil depths under grassland [541 µg/g (0–5 cm), 468 µg/g (5–10 cm)]. On the other hand, inorganic S concentrations were significantly higher in soil under forest at all depths compared with grassland. The inorganic S concentration in soil under grassland increased with depth, but there was no significant difference observed at different depths under forest. The decrease in organic S [and organic carbon (C)] in soil under forest was due to the enhanced mineralisation of organic components. The accumulation of inorganic S in the soil profile under forest was mainly attributed to enhanced mineralisation, although decreased leaching, increased sulfate-S adsorption, and increased atmospheric inputs by canopy interception of aerosols could have contributed. Microbial biomass C and S and arylsulfatase activity were higher in topsoil under grassland than forest. Lower arylsulfatase activities under forest compared with grassland at the time of sampling suggest that mineralisation of organic S under forest was not currently mediated primarily by enzyme activity, although enzyme activity may have been important at earlier stages of forest development. Arylsulfatase activity was significantly correlated with soil organic C, water-soluble C, microbial biomass C, total S, organic S, and microbial biomass S in soil under grassland and forest. Significant concentrations of organic S and microbial biomass S were present in the forest floor (litter and fermentation layers). These pools would be important for S cycling and availability in forest ecosystems. S mineralisation, S fractions, microbial biomass S, microbial biomass C.

Effects of mixing radiata pine needles and understory litters on decomposition and nutrients release

A microcosm experiment was conducted to understand the impacts of mixing radiata pine (Pinus radiata D. Don) needle litter and understory (gorse—Ulex europaeus L., broom—Cytisus scoparius L., bracken—Pteridium aquilinum L., and lotus—Lotus pedunculatus L.) litter materials on decomposition and nutrient release dynamics. Mixing of pine needle litter with understory litter material had significant impacts on both the rate of decomposition and nutrient release patterns of pine litter as well as that of the understory species. Incubation in microcosms over 10 months resulted in significantly lower mass loss of radiata pine needle litter mixed with broom and lotus litters (35.8±8.4 and 41.3±0.8%, respectively) than pure pine needle litter (63.5±2.3%). Mixing with pine needle litter significantly increased the mass loss of broom (53.1±6.1%) compared to that of pure broom (30.2±1.0%). Significant transfers of nutrients, especially of magnesium and potassium, were observed in litter mixture treatments. Concentration of K in litter materials was found to be the most limiting factor for the decomposing microorganisms in the present study. The findings of this study suggest that management of understory litter composition via weed control could be used to manipulate carbon turnover and nutrient release in the forest floor. Also, initial selection of understory species will be important and could be managed.