Phosphorus in Agriculture: A Review of Results from 175 Years of Research at Rothamsted, UK.

Insight into the role of phosphorus (P) in soil fertility and crop nutrition at Rothamsted, UK, and its involvement in associated environmental issues, has come from long-term field experiments initially started by J. B. Lawes in 1843 and continued by others, together with experiments on different soils. Results from the 1940s confirmed that residues of P applied in fertilizers and manures build up reserves of P in soil. There is a strong relationship between crop yield and plant-available P (Olsen P), and a critical level of Olsen P can be determined. For soils near the critical level, P-use efficiency is high when the P applied and offtake by the crop is nearly equal. Soil inorganic P is associated with various soil components and is held there with a range of bonding energies so that when no P is applied, the decline in Olsen P follows a smooth curve. We conceptualize inorganic soil P as being in four pools of vastly varying size, availability for uptake, and extractability by reagents used in routine soil analysis, and with reversible transfer of P between pools. For very disparate soils at Rothamsted and in the United States, there is a strong relationship between the change in Olsen P and P removal/input ratios, suggesting an underlying similarity in inorganic P behavior. Maintaining soil near the critical level should optimize yield and the use of the global P resource while minimizing the risk of transfer of large amounts of P to the aquatic environment.

The importance of long-term experiments in agriculture: their management to ensure continued crop production and soil fertility; the Rothamsted experience.

Long-term field experiments that test a range of treatments and are intended to assess the sustainability of crop production, and thus food security, must be managed actively to identify any treatment that is failing to maintain or increase yields. Once identified, carefully considered changes can be made to the treatment or management, and if they are successful yields will change. If suitable changes cannot be made to an experiment to ensure its continued relevance to sustainable crop production, then it should be stopped. Long-term experiments have many other uses. They provide a field resource and samples for research on plant and soil processes and properties, especially those properties where change occurs slowly and affects soil fertility. Archived samples of all inputs and outputs are an invaluable source of material for future research, and data from current and archived samples can be used to develop models to describe soil and plant processes. Such changes and uses in the Rothamsted experiments are described, and demonstrate that with the appropriate crop, soil and management, acceptable yields can be maintained for many years, with either organic manure or inorganic fertilizers. Highlights: Long-term experiments demonstrate sustainability and increases in crop yield when managed to optimize soil fertility.Shifting individual response curves into coincidence increases understanding of the factors involved.Changes in inorganic and organic pollutants in archived crop and soil samples are related to inputs over time.Models describing soil processes are developed from current and archived soil data.

Changes in soil organic matter over 70 years in continuous arable and ley-arable rotations on a sandy loam soil in England.

The sequestration in soil of organic carbon (SOC) derived from atmospheric carbon dioxide (CO2) by replacing arable crops with leys, has been measured over 70 years on a sandy loam soil. The experiment was designed initially to test the effect of leys on the yields of arable crops. A 3-year grazed grass with clover (grass + clover) ley in a 5-year rotation with arable crops increased percentage organic carbon (%OC) in the top 25 cm of the soil from 0.98 to 1.23 in 28 years, but with little further increase during the next 40 years with all-grass leys given fertilizer nitrogen (N). In this second period, OC inputs were balanced by losses, suggesting that about 1.3% OC might be near the equilibrium content for this rotation. Including 3-year lucerne (Medicago sativa) leys had little effect on %OC over 28 years, but after changing to grass + clover leys, %OC increased to 1.24 during the next 40 years. Eight-year leys (all grass with N or grass + clover) in 10-year rotations with arable crops were started in the 1970s, and after three rotations %OC had increased to ca. 1.40 in 2000-2009. Over 70 years, %OC declined from 0.98 to 0.94 in an all-arable rotation with mainly cereals and to 0.82 with more root crops. Applications of 38 t ha-1 farmyard manure (FYM) every fifth year increased %OC by 0.13% by the mid-1960s when applications ceased. Soil treated with FYM still contained 0.10% more OC in 2000-2009. Changes in the amount of OC have been modelled with RothC-26.3 and estimated inputs of C for selected rotations. Little of the OC input during the 70 years has been retained; most was retained in the grazed ley rotation, but 9 t ha-1 only of a total input of 189 t ha-1. In other rotations more than 98% of the total OC input was lost. Despite large losses of C, annual increases in OC of 4‰ are possible on this soil type with the inclusion of grass or grass + clover leys or the application of FYM, but only for a limited period. Such increases in SOC might help to limit increases in atmospheric CO2. HIGHLIGHTS: Can leys sequester significant amounts of atmospheric CO 2 in SOM and contribute to the 4‰ initiative?Changes in the percentage and amount of OC were measured and modelled over 70 years and OC losses estimated.Three-year grass or grass + clover leys increased %OC, but only to an equilibrium level that was then maintained.Despite large losses, sequestering CO 2-C at 4‰ year-1 by growing grass or grass + clover leys is possible.

Grassland biodiversity bounces back from long-term nitrogen addition.

The negative effect of increasing atmospheric nitrogen (N) pollution on grassland biodiversity is now incontrovertible. However, the recent introduction of cleaner technologies in the UK has led to reductions in the emissions of nitrogen oxides, with concomitant decreases in N deposition. The degree to which grassland biodiversity can be expected to 'bounce back' in response to these improvements in air quality is uncertain, with a suggestion that long-term chronic N addition may lead to an alternative low biodiversity state. Here we present evidence from the 160-year-old Park Grass Experiment at Rothamsted Research, UK, that shows a positive response of biodiversity to reducing N addition from either atmospheric pollution or fertilizers. The proportion of legumes, species richness and diversity increased across the experiment between 1991 and 2012 as both wet and dry N deposition declined. Plots that stopped receiving inorganic N fertilizer in 1989 recovered much of the diversity that had been lost, especially if limed. There was no evidence that chronic N addition has resulted in an alternative low biodiversity state on the Park Grass plots, except where there has been extreme acidification, although it is likely that the recovery of plant communities has been facilitated by the twice-yearly mowing and removal of biomass. This may also explain why a comparable response of plant communities to reduced N inputs has yet to be observed in the wider landscape.

Soil moisture mediates association between the winter North Atlantic Oscillation and summer growth in the Park Grass Experiment.

Several aspects of terrestrial ecosystems are known to be associated with the North Atlantic Oscillation (NAO) through effects of the NAO on winter climate, but recently the winter NAO has also been shown to be correlated with the following summer climate, including drought. Since drought is a major factor determining grassland primary productivity, the hypothesis was tested that the winter NAO is associated with summer herbage growth through soil moisture availability, using data from the Park Grass Experiment at Rothamsted, UK between 1960 and 1999. The herbage growth rate, mean daily rainfall, mean daily potential evapotranspiration (PE) and the mean and maximum potential soil moisture deficit (PSMD) were calculated between the two annual cuts in early summer and autumn for the unlimed, unfertilized plots. Mean and maximum PSMD were more highly correlated than rainfall or PE with herbage growth rate. Regression analysis showed that the natural logarithm of the herbage growth rate approximately halved for a 250 mm increase in maximum PSMD over the range 50-485 mm. The maximum PSMD was moderately correlated with the preceding winter NAO, with a positive winter NAO index associated with greater maximum PSMD. A positive winter NAO index was also associated with low herbage growth rate, accounting for 22% of the interannual variation in the growth rate. It was concluded that the association between the winter NAO and summer herbage growth rate is mediated by the PSMD in summer.

Mechanisms of phosphorus solubilisation in a limed soil as a function of pH.

Phosphorus (P) quantity-intensity relationships are central to the solubility and release of P from soil to water. Relationships between P extractable by 0.5 M NaHCO extractable P (Olsen P; quantity, Q) and P extractable by 0.01 M CaCl(2) (CaCl(2)-P; possible predictor of soil solution or drainage water P; intensity, I) are curvilinear: above a certain Olsen P concentration, CaCl(2)-P becomes much more soluble than when below it. Aluminium-, Fe- and Ca-P forms (extractable by Olsen's reagent) are thought to control P solubility. Thus, our objectives were to identify P forms in equilibrium with CaCl(2)-P via solubility equilibrium experiments, and the behaviour of CaCl(2)-P in relation to Al, Fe and Ca associated P, determined with 31P high power decoupling magic angle spinning nuclear magnetic resonance spectroscopy (31P HPDec/MAS NMR). Results indicated that two Q-I relationships occurred, one for soils above pH 5.8, and the other for soils below pH 5.8. Above pH 5.8, soils were saturated with respect to hydroxyapatite (Ca(5)(PO(4))(3)OH) and undersaturated with respect to beta-tricalcium phosphate (beta-Ca(3)(PO(4))(2)), while log ion-activity products showed that all soils and pHs were either saturated or in equilibrium with variscite (AlPO(4).2H(2)O) or its amorphous analogue. Using 31P HPDec/MAS NMR, Ca-P was best correlated with CaCl(2)-P in soils above pH 5.8, and with Al-P in soils below this pH. This study demonstrates the value of solid-state NMR in conjunction with wet chemical techniques for the study of labile P and P loss from pasture soils with a wide range of managements.

Analysis of potentially mobile phosphorus in arable soils using solid state nuclear magnetic resonance.

In many intensive agroecosystems continued inputs of phosphorus (P) over many years can significantly increase soil P concentrations and the risk of P loss to surface waters. For this study we used solid-state 31P nuclear magnetic resonance (NMR) spectroscopy, high-power decoupling with magic angle spinning (HPDec-MAS) NMR, and cross polarization with magic angle spinning (CP-MAS) NMR to determine the chemical nature of potentially mobile P associated with aluminum (Al) and calcium (Ca) in selected arable soils. Three soils with a range of bicarbonate-extractable Olsen P concentrations (40-102 mg P kg(-1)) were obtained from a long-term field experiment on continuous root crops at Rothamsted, UK, established in 1843 (sampled 1958). This soil has a threshold or change point at 59 mg Olsen P kg(-1), above which potentially mobile P (as determined by extraction with water or 0.01 M CaCl2) increases much more per unit increase in Olsen P than below this point. Results showed that CaCl2 and water preferentially extracted Al-P and Ca-P forms, respectively, from the soils. Comparison among the different soils also indicated that potentially mobile P above the threshold was largely present as a combination of soluble and loosely adsorbed (protonated-cross polarized) P forms largely associated with Ca, such as monetite (CaHPO4) and dicalcium phosphate dihydrate (CaHPO4-2H2O), and some Al-associated P as wavellite. The findings of this study demonstrate that solid-state NMR has the potential to provide accurate information on the chemical nature of soil P species and their potential mobility.