P. R. Poulton

Simulating trends in soil organic carbon in long-term experiments using RothC-26.3

Abstract As part of a model evaluation exercise, RothC-26.3, a model for the turnover of organic carbon in non-waterlogged soils, was fitted to measurements of organic carbon from 18 different experimental treatments on 6 long-term experimental sites in Germany, England, the USA, the Czech Republic and Australia. In the fitting process, the model was first run with an annual return of plant C that had been selected iteratively to give the carbon content of the soil at the start of each experiment. This was done for the soil and climate of each site. If the radiocarbon content of the soil organic matter was known, the inert organic carbon (IOM) content of the soil was also calculated for the start of the experiment. Using these carbon and radiocarbon contents as a starting point, the model was then run for each of the experimental treatments to be fitted, using iteratively selected values for the annual return of plant materials to the soil. The value used for each treatment was selected to optimise the fit between modelled and measured data over the whole experimental period: fitting was done by eye. Thus fitted, RothC-26.3 gave an acceptable approximation to the measurements for 14 of the treatments, bearing in mind the experimental errors in measuring soil organic carbon on a per hectare basis. With four of the treatments (Highfield Bare Fallow, Park Grass plot 13d, Ruzyně farmyard manure plot and Tamworth rotation 5), the fit was less satisfactory.

Soluble organic nitrogen in agricultural soils

Abstract The existence of soluble organic forms of N in rain and drainage waters has been known for many years, but these have not been generally regarded as significant pools of N in agricultural soils. We review the size and function of both soluble organic N extracted from soils (SON) and dissolved organic N present in soil solution and drainage waters (DON) in arable agricultural soils. SON is of the same order of magnitude as mineral N and of equal size in many cases; 20–30 kg SON-N ha–1 is present in a wide range of arable agricultural soils from England. Its dynamics are affected by mineralisation, immobilisation, leaching and plant uptake in the same way as those of mineral N, but its pool size is more constant than that of mineral N. DON can be sampled from soil solution using suction cups and collected in drainage waters. Significant amounts of DON are leached, but this comprises only about one-tenth of the SON extracted from the same soil. Leached DON may take with it nutrients, chelated or complexed metals and pesticides. SON/DON is clearly an important pool in N transformations and plant uptake, but there are still many gaps in our understanding.

Organic geochemical studies of soils from the Rothamsted Classical Experiments - I. Total lipid extracts, solvent insoluble residues and humic acids from Broadbalk Wilderness

Abstract Total lipid extracts and insoluble organic matter, i.e. solvent insoluble matter and humic acids, were studied from soil samples taken from the three adjacent plots comprising the Broadbalk Wilderness at Rothamsted Experimental Station, Harpenden, Hertfordshire, U.K. Analyses involved high-temperature gas chromatography (HT-GC) and HT-GC-mass spectrometry (HT-GC-MS) to investigate trimethylsilylated total lipid extracts and Curie-point pyrolysis-GC (Py-GC) and Py-GC-MS to investigate solvent insoluble fractions. The plots were chosen specifically for their different types of vegetation cover. Samples of the vegetation were examined in parallel with the underlying soils in an effort to follow the fate of the major plant components in soil. The application of HT-GC and HT-GC-MS allowed changes in high molecular weight lipids, particularly intact acyl lipids, such as triacylglycerols, wax esters, steryl and triterpenyl esters, to be studied in leaf and soil extracts. The total lipid extracts of the soil samples from the wooded area were dominated by the input from leaf-derived lipids. The lipid extracts of soils from the grazed and stubbed areas were markedly different from those from the wooded area, and reflected the mixed vegetation cover dominated by grass species. In marked contrast, the pyrolysis data from the insoluble organic matter and humic fractions of the soils did not reflect the composition of the lignin comprising the overlying vegetation, but rather showed evidence of amino acid moieties probably present as polypeptides. The absence of the lignin signal is possibly due to rapid diagenetic changes presumed to be influenced by the slightly alkaline pH of the soil. The ability to recover recognizable chemical signals from soil lipids has important implications for archaeological investigations aimed at revealing temporal changes in vegetation cover and/or differences in land use at specific site locations.

Evidence of decreasing mineral density in wheat grain over the last 160 years.

Wheat is an important source of minerals such as iron, zinc, copper and magnesium in the UK diet. The dietary intake of these nutrients has fallen in recent years because of a combination of reduced energy requirements associated with sedentary lifestyles and changes in dietary patterns associated with lower micronutrient density in the diet. Recent publications using data from food composition tables indicate a downward trend in the mineral content of foods and it has been suggested that intensive farming practices may result in soil depletion of minerals. The aim of our study was to evaluate changes in the mineral concentration of wheat using a robust approach to establish whether trends are due to plant factors (e.g. cultivar, yield) or changes in soil nutrient concentration. The mineral concentration of archived wheat grain and soil samples from the Broadbalk Wheat Experiment (established in 1843 at Rothamsted, UK) was determined and trends over time examined in relation to cultivar, yield, and harvest index. The concentrations of zinc, iron, copper and magnesium remained stable between 1845 and the mid 1960s, but since then have decreased significantly, which coincided with the introduction of semi-dwarf, high-yielding cultivars. In comparison, the concentrations in soil have either increased or remained stable. Similarly decreasing trends were observed in different treatments receiving no fertilizers, inorganic fertilizers or organic manure. Multiple regression analysis showed that both increasing yield and harvest index were highly significant factors that explained the downward trend in grain mineral concentration.

Simulating trends in soil organic carbon in long-term experiments using the century model

Abstract This paper describes Century Soil Organic Matter (SOM) Model simulations of seven long-term data sets that are the subject of this special issue. We found that Century successfully simulates SOM C across a variety of land use and climate types. Simulations of SOM were most successful in grass and crop systems. This exercise highlights a structural limitation of Century in simulating SOM in a forest with a developed litter layer. Simulations of tree biomass distributions, however, were generally successful. The model failed to capture extreme values of yield and N offtake, although annual averages were quite similar between observations and simulations, leading to reasonable estimates of SOM C. Average yields and SOM were generally higher in amended treatments or rotations including an N-fixing component such as alfalfa. The model successfully predicted SOM dynamics across climates, land use types, and treatments. This suggests that Century is a useful tool for ecosystem studies, particularly those focused on SOM dynamics.

Organic geochemical studies of soils from the Rothamsted classical experiments - V. The fate of lipids in different long-term experiments.

Lipid extracts from four long-term experiments (Broadbalk Wilderness, Geescroft Wilderness, Hoosfield Spring Barley and Park Grass) were analysed using a combination of gas chromatography, gas chromatography‐mass spectrometry and gas chromatography‐combustion‐isotope ratio mass spectrometry. The lipid content of the primary organic inputs for each soil werealso analysed inorder toassess the early diagenetic fate ofthe variouscompound classespresent.SoilpH was observed to, either directly or indirectly, have a significant eAect on lipids with a relative increase in abundance of n-alkanes at higher pH (7.31) and a large relative increase in n-alkanoic and o-hydroxy acids at low pH (3.74). Triacylglycerols exhibited severe losses irrespective of pH. In an arable soil, n-alkanoic acids showed a temporal decrease in concentration whilst levels of n-alkanols remained static, the diAerence was ascribed to a more rapid turnover and possible leachate migration of the n-alkanoic acids. The phytosterol, sitosterol, was observed to rapidly diminish in soils possibly as a result of assimilation by soil dwelling invertebrates. Analysis of 5b-stigmastanol (a faecal biomarker) showed that it remainedat levels indicativeofmanuringeven after 113 years. Furthermore, analysis of 5b-stanyl esters revealed a manuringsignal even more persistent than that exhibited by the free stanols. Knowledge of the biogeochemical cycling of lipids in the soil environment will help facilitate understanding of the processes which underpin carbon cycling in soils. # 2000 Elsevier Science Ltd. All rights reserved.

Organic geochemical studies of soils from the Rothamsted classical experiments-IV. Preliminary results from a study of the effect of soil pH on organic matter decay

Total lipid extracts and solvent insoluble organic matter in soils from the Park Grass Exper- iment at Rothamsted Experimental Station, Harpenden, U.K. were studied to determine the eAect of pH on the preservation/degradation of plant derived biomolecules. Analyses involved high temperature- gas chromatography (HT-GC), HT-GC-mass spectrometry (HT-GC-MS), GC combustion-isotope ratio MS (GCC-IRMS) and flash pyrolysis-GC (Py-GC) and Py-GC-MS. The plots selected for study have pH values ranging from 3.7 to 7.3, with acidic soils exhibiting two distinct horizons (i.e. humic rich top layer and mineral soil). The total lipid extracts of the soil samples with low pH exhib- ited higher relative abundances of long-chain (>C20) organic acids believed to be derived largely from oxidation of plant lipids. The vegetation signature in the low molecular weight fraction is only retained in the humic rich top layer. The signal in the mineral layer is believed to derive primarily from previous vegetation. Compound specific stable carbon isotope (d 13 C) measurements of long-chain n-alkanols are considered to reflect diAerences in the rate of incorporation of plant lipids into the humic top layer re- lated to the grass species dominating the standing vegetation. In the soil samples of low pH, lignin con- tributes to the high molecular weight fraction of the humic layer. In contrast, the mineral layer of the same soil shows little evidence of intact lignin, but is instead dominated by amino acid pyrolysis pro- ducts, probably deriving from (degraded) polypeptides. The pyrolysates of the mineral soils of high pH yield a distribution of products similar to that found in the deeper layer of the low pH samples but with evidence of lignin derived moieties. Overall, soil pH was found to have a significant eAect on the preservation of higher plant derived biomolecules including ligno-cellulose. # 1998 Elsevier Science Ltd. All rights reserved

Simulating trends in soil organic carbon in long-term experiments using the DNDC model

Abstract Simulations of long-term (> 20 year) soil organic carbon (SOC) dynamics by the DNDC model were compared with field observations at 11 plots in 5 field stations in Europe and Australia. The exercise was part of a NATO-sponsored workshop on long-term monitoring and modeling of soil organic matter. Eight of the eleven plots were cultivated cropland and three were grassland (harvested for hay). There were a range of fertilizer and manure treatments, as well as crop rotation sequences. Significant loss in SOC was observed at two plots in Australia where a grassland had been converted to cultivated cropland in 1925. Both field data and model simulations showed the plots reaching a new SOC equilibrium at about 44% of the 1925 levels. Equilibrium levels depended on crop rotation sequence, with higher SOC for the plot with less frequent fallowing. At one permanent grassland site at Rothamsted, UK, a large decline and recovery in SOC was observed in the field, but not in the model simulation. For all other cases, both field and model data showed relatively small changes in SOC, though field data tended to be more variable, perhaps due to variability in both crop and weed yield, and in residue management. Mean percent differences between simulated and measured SOC were 0.07% or less (as percent by weight, kilogram SOC/kilogram soil) for all but one of the plots simulated.

Influence of soil type, crop management and weather on the recovery of 15N-labelled fertilizer applied to winter wheat in spring

15 N-labelled fertilizer was applied, in spring, to winter wheat crops in nine experiments in eastern England over a period of 4 years. Five were on Batcombe Series silty clay loam, two on Beccles Series sandy clay loam (with a mole-drained clay subsoil) and two on Cottenham Series sandy loam. In three of the experiments, different rates of fertilizer N were applied (up to 234 kg N/ha); in the others, a single rate (between 140 and 230 kg/ha) was used. Recovery of fertilizer N in the above-ground crop (grain, chaff, straw and stubble) ranged from 46 to 87% (mean 68%). The quantity of fertilizer N retained in the soil at harvest was remarkably constant between different experiments, averaging 18% where labelled N was applied as 15 NH 4 15 NO 3 , but less (7–14%) where K 16 NO 3 was applied. Of the labelled N present in soil to a depth of 70 cm, 84–88% was within the cultivated layer (0–23 cm). L70 = 5(± 1 63) + 0·264(±00352) R 3 accounted for 73% of the variation in the data where: L 70 = percentage loss of fertilizer N from the crop: soil system, defined as percentage of labelled N not recovered in crop or in soil to a depth of 70 cm at the time of harvest; R 3 = rainfall (in mm) in the 3 weeks following application of N fertilizer. There was a tendency for percentage loss of fertilizer N to be greater when a quantity of N in excess of that required for maximum grain yield was applied. However, a multiple regression relating loss both to rainfall and to quantity of N applied accounted for no more variance than the regression involving rainfall alone. In one experiment, early and late sowing were compared on the first wheat crop that followed oats. The loss of N from the early-sown crop, given fertilizer N late in spring, was only 4% compared with 26 % from the later-sown crop given N at the same time, so that sowing date had a marked effect on the loss of spring-applied fertilizer N. Uptake of unlabelled N, derived from mineralization of organic N in soil, autumn-applied N (where given) and from atmospheric inputs, was 130 kg/ha when wheat followed potatoes or beans on soil containing c. 0·15 % total N. Unlabelled N accounted for 20–50% of the total N content of fertilized crops at harvest. About 50% of this unlabelled N had already been taken up by the time of fertilizer application in spring and the final quantity was closely correlated with the amount present in the crop at this time. Applications of labelled fertilizer N tended to increase uptake of unlabelled N by 10–20 kg/ha, compared to controls receiving no N fertilizer. This was probably due to pool substitution, i.e. labelled inorganic N standing proxy for unlabelled inorganic N that would otherwise have been immobilized or denitrified.

Effects of season, soil type and cropping on recoveries, residues and losses of 15N-labelled fertilizer applied to arable crops in spring

15 N-labelled fertilizer was applied in spring to winter wheat, winter oilseed rape, potatoes, sugarbeet and spring beans in field experiments done in 1987 and 1988 in SE England on four contrasting soil types – a silty clay loam, a chalky loam, a sandy loam and a heavy clay. The 15 N-labelled fertilizers were applied at recommended rates; for oilseed rape, a two-thirds rate was also tested. Whole-crop recoveries of labelled nitrogen averaged 52% for winter wheat, 45% for oilseed rape, 61% for potatoes and 61% for sugarbeet. Spring beans, which received only 2·5 kg ha −1 of labelled N, recovered 26%. Removals of 15 N-labelled fertilizer N in the harvested products were rather less, averaging 32, 25, 49, 27 and 13% in wheat grain, rape seed, potato tubers, beet root and bean grain, respectively. Crop residues were either baled and removed, as with wheat and rape straw, or were flailed or ‘topped’ and left on the soil surface, as was the case with potato tops and sugarbeet tops. Wheat stubble and rape stubble, together with leaf litter and weeds, were incorporated after harvest. The ploughing in of crop residues returned 4–35% of the original nitrogen fertilizer application to the soil, in addition to that which already remained at harvest, which averaged 24, 29 and 25% of that applied to winter wheat, oilseed rape and sugarbeet respectively. Less remained at harvest after potatoes ( c . 21%) and more after spring beans ( c . 49%). Most of the labelled residue remained in the top-soil (0–23cm) layer. 15 N-labelled fertilizer unaccounted for in crop and soil (0–100 cm) at harvest of winter wheat, oilseed rape, potatoes, sugarbeet and spring beans averaged 23, 25, 19, 14 and 26% of that applied, respectively. Gaseous losses of fertilizer N by denitrification were probably greater following applications to winter wheat and oilseed rape, where the N was applied earlier (and the soils were wetter) than with potatoes and sugarbeet. Consequently, it may well be advantageous to delay the application of fertilizer N to winter wheat and oilseed rape if the soil is wet. Total inorganic N (labelled and unlabelled) in soils (0–100 cm) following harvest of potatoes given 15 N-labelled fertilizer in spring averaged 70 kg N ha −1 and was often greater than after the corresponding crops of winter wheat and oilseed rape, which averaged 53 kg N ha −1 and 49 kg N ha −1 , respectively. On average, 91 kg ha −1 of inorganic N was found in soil (0–100 cm) following spring beans. Least inorganic N remained in the soil following sugarbeet, averaging only 19 kg N ha −1 . The risk of nitrate leaching in the following winter, based on that which remained in the soil at harvest, ranked in decreasing order, was: spring beans=potatoes>oilseed rape=winter wheat>sugarbeet. On average, only 2·9% of the labelled fertilizer applied to winter wheat and oilseed rape remained in the soil (0–100 cm) as inorganic N (NO − 3 +NH + 4 ) at harvest; with sugarbeet only 1·1% remained. In most cases c . 10% of the mineral N present in the soil at this time was derived from the nitrogen fertilizer applied to arable crops in spring. However, substantially more ( c . 21%) was derived from fertilizer following harvest of winter wheat infected with take-all ( Gaeumannomyces graminis var. tritici ) and after potatoes. With winter wheat and sugarbeet, withholding fertilizer N had little effect on the total quantity of inorganic N present in the soil profile at harvest, but with oilseed rape and potatoes there was a decrease of, on average, 38 and 50%, respectively. A decrease in the amount of nitrogen applied to winter wheat and sugarbeet in spring would therefore not significantly decrease the quantity of nitrate at risk to leaching during the following autumn and winter, but may be more effective with rape and potatoes. However, if wheat growth is severely impaired by take-all, significant amounts of fertilizer-derived nitrate will remain in the soil at harvest, at risk to leaching.