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Soil phosphorus (P) reserves, built up over decades of intensive agriculture, may account for most of the crop P uptake, provided adequate supply of other plant nutrients. Whether crops grown on soils with reduced supply of other nutrients obtain similar use-efficiency of soil P reserves remains unclear. In treatments of the Askov Long-Term Experiment (initiated in 1894 on light sandy loam), we quantified changes in soil total P and in plant-available P (Olsen P, water extractable P and P offtake in wheat grains) when P-depleted soil started receiving P in rock phosphate and when P application to soil with moderate P levels ceased during 1997-2017. Additionally we studied treatments with soil kept unfertilised for >100 years and with soil first being P depleted and then exposed to surplus dressings of P, nitrogen (N) and potassium in cattle manure. For soil kept unfertilised for >100 years, average grain P off take was 6 kg [ha.sup.-1] and Olsen P averaged 4.6 mg [kg.sup.-1], representing the lower asymptotic level of plant-available P. Adding igneous rock phosphate to severely P-depleted soil with no N fertilisation had little effect on Olsen P, water extractable P (Pw), grain yields and P offtake. For soils with moderate levels of available P, withholding P application for 20 years reduced contents of Olsen P by 56% (from 16 to 7 mg P [kg.sup.-1]) and of Pw by 63% (from 4.5 to 1.7 mg P [kg.sup.-1]). However, the level of plant-available P was still above that of unfertilised soil. Application of animal manure to P-depleted soil gradually raised soil P availability, grain yield and P offtake, but it took 20 years to restore levels of plant-available P. Our study suggests symmetry between rates of depletion and accumulation of plant-available P in soil. Additional keywords: cattle manure, Denmark, rock phosphate, soil P reserves, Typic Hapludalf, winter wheat. Received 1 August 2019, accepted 18 October 2019, published online 19 November 2019, Introduction The availability and use-efficiency of phosphorus (P) in global agriculture remains high on the agenda with some 90% of the mined non-renewable resources of rock phosphate used for production [...]

Nutrient management affects not only crop productivity and environmental quality, but also soil physical properties related to soil tilth. Previous studies on soil physical properties have focussed on effects of fertiliser type, whereas the effect of fertiliser rate has been neglected. We examined the impact of no fertilisation (UNF) and different rates of mineral fertiliser (1/2 NPK and 1NPK) and animal manure (1 '/2AM) on an ensemble of soil physical characteristics, with the amount of fertiliser added at level 1 corresponding to the standard rate of plant nutrients for a given crop. Soil was from the Askov long-term field experiment, initiated in 1894 on a hard-setting sandy loam. We assessed clay dispersibility, wetstability of aggregates, aggregate strength, bulk soil strength and soil pore characteristics. The soils receiving 1NPK and 1 1/2AM had similar soil physical properties, the only differences being a wider range in the optimum water content for tillage and more plant-available water in the soil amended with 1 1/2AM. Suboptimal fertiliser rates (UNF and 1/2NPK.) increased clay dispersibility, soil cohesion and bulk density, and reduced aggregate stability. The physical properties of soils exposed to suboptimal fertilisation indicate that the level of soil organic matter, including active organic binding and bonding materials, has become critically low due to reduced inputs of crop residues. While long-term suboptimal fertilisation compromises soil physical properties, crop-yield-optimised rates of mineral fertilisers and animal manure appear to sustain several soil physical properties equally well. Additional keywords: manure, mineral fertiliser, soil organic carbon. Received 16 August 2016, accepted 24 October 2016, published online 24 November 2016, Introduction During recent decades, a more comprehensive concept of crop production sustainability has evolved within the European Union (Jones et al. 2012), emphasising that sustainable management involves protection of the [...]

Background: A high use‐efficiency of fertilizer N remains essential to sustain high crop productivity with low environmental impact. However, little is known on the long‐term lability of mineral fertilizer N. Aims: To quantify crop uptake and leaching of 15N‐labelled mineral fertilizer that has been retained in an agricultural soil for 25–30 years in crops with variable growing season. Methods: A field plot received 15N‐labelled mineral fertilizers over a period of 5 years and was then kept under arable cropping for 12 years. After relocation to 16 lysimeters, the topsoil grew set‐aside grassland for the next 13 years. Then crop uptakes and leaching losses of 15N remaining in soil was tested over a 2‐year period by either converting set‐aside grass to production grassland, or by replacing it with spring barley (+/− autumn cover crop) or vegetation‐free fallow. All treatments received unlabelled mineral N fertilizers. Results: Crop uptake and leaching of 15N were generally highest in the first test year after termination of the set‐aside. The leaching of residual 15N in soil declined in the order: vegetation‐free soil (4.7%), spring barley (1.9%), spring barley + cover crop (0.7%) and production grassland (0.2%). Corresponding losses for the second leaching period were 2.7%, 0.9%, 0.4% and 0.06%. There was a fixed relationship between leaching losses of 15N and total N. Conclusions: After residing in soil for 25–30 years, the lability of labelled mineral N fertilizer residues appeared slightly higher than the lability of bulk soil N. Autumn vegetation was crucial for reducing leaching losses. [ABSTRACT FROM AUTHOR]

Foxtail and broomcorn millets are the most important crops in northern China since the early Neolithic. However, little evidence is available on how people managed these two crops in the past, especially in prehistory. Previous research on major C3 crops in western Eurasia demonstrated the potential of stable carbon and nitrogen isotope analysis of charred archaeobotanical remains to reveal the management of water and manure, respectively. Here, we evaluate the feasibility of a similar approach to C4 millets. Foxtail and broomcorn millet plants grown in pots in a greenhouse under different manuring and watering regimes were analysed to test the effects of management on stable carbon and nitrogen isotope values of grains. Stable nitrogen isotope values of both millets increased as manuring level increased, ranging from 1.7‰ to 5.8‰ in different conditions; hence, it appears a feasible tool to identify manuring practices, in agreement with results from recent field studies. However, the two millets exhibit opposing trends in stable carbon isotope values as watering level increased. The shift in stable carbon isotope values of millets is also smaller than that observed in wheat grown in the same experimental environment, making it difficult to identify millet water status archaeologically. In addition, we charred millet grains at different temperatures and for varying durations to replicate macro-botanical remains recovered archaeologically, and to evaluate the offsets in carbon and nitrogen isotope values induced by charring. We found that the stable nitrogen isotope values of foxtail millet and broomcorn millet can shift up to 1-2 ‰ when charred, while the stable carbon isotope values change less than 0.3‰. Overall, we demonstrate that stable nitrogen isotope values of charred foxtail and broomcorn millet seeds could provide insight into past field management practices, and both carbon and nitrogen isotope values can together inform palaeodietary reconstruction. [ABSTRACT FROM AUTHOR]

To elucidate the mechanisms responsible for the long-term retention of soil C, soil samples were retrieved from a paleosolic A horizon ([M.sub.bur]) buried underneath a Bronze Age mound and from waterlogged anaerobic ([M.sub.an]) and aerobic zones ([M.sub.ae]) within the mound. For comparison with a modern soil, samples were taken from the surrounding arable field (Ar). The soils were characterized by cross-polarization magic angle spinning [sup.13]C nuclear magnetic resonance spectroscopy, by phospholipid fatty acid (PLFA) analysis, and by C[O.sub.2] production during incubation. At sampling, the Ar, [M.sub.an], [M.sub.ae], and [M.sub.bur] soils contained 22.6, 17.9, 6.5, and 6.2 g C [kg.sup.-1] soil, respectively. Compared with the Ar and [M.sub.an] soils, the [M.sub.bur] and [M.sub.ae] soils were depleted in alkyl C, N-alkyl C, and O-alkyl C and enriched in aromatic and carboxylic groups. The content of PLFA in the mound soils was much lower than normally found for arable soils and the ability of the deprived biomass to decompose glucose was retarded and included a 2- to 3-wk lag phase. The lability of C left under aerobic conditions for more than three millennia did not differ significantly from that of C in the contemporary Ar soil. Although the chemical nature of the C in the Ar and [M.sub.an] soils was similar, C that had resided under anaerobic conditions was less labile than C from the arable soil. We consider the persistence of C in the unwaterlogged parts of the burial mound to result from physical restrictions imposed on gas exchange and water availability, and thus restricted microbial activity, rather than from biochemical recalcitrance of the remaining C. Abbreviations: NMR, nuclear magnetic resonance; OM, organic matter; PLFA, phospholipid fatty acid.

Quantification of in situ soil microbial activity is indispensable to improve manipulation of nutrient turnover in soil and optimize crop nutrient supply. We sampled 100-[cm.sup.3] cores of undisturbed arable soil at three locations along a naturally occurring clay gradient (L1: 11% clay; L3: 22% clay; L5: 34% clay). The cores were drained to seven different matric potentials in the range -15 to -1500 hPa and gas diffusion determined prior to a 4-wk incubation at 20[degrees]C in the dark. For all soils the net nitrification increased with water content to a maximum (L1, 12.1 [micro]g N[O.sub.3]-N [g.sup.-1] soil; L3, 10.3 [micro]g N[O.sub.3]-N [g.sup.-1] soil; and L5, 8.2 [micro]g N[O.sub.3]-N [g.sup.-1] soil) and then decreased with further increase in water content. The water content at maximum nitrification was 0.26, 0.37, and 0.42 [m.sup.3] [m.sup.-3], respectively. Calculations of water-filled pore space (WFPS) did not normalize soil type differences in optima for microbial activity. The matric potential at peak net nitrification was -140, -170, and -430 hPa, respectively. No single correlation between C[O.sub.2] evolution and soil water content existed across soil types. The relative solute diffusivity estimated by recently developed models offered a better description of C[O.sub.2] evolution. The relative gas diffusivity was a better predictor of the increase in net nitrification than was the soil air content. A conceptual model balancing the effects of solute and gas diffusivity indicated that the relative trend in the observed optima of water contents across soil types was as expected. We advocate the use of the conceptual model including soil type dependent expressions for solute and gas diffusivity in future studies of aerobic microbial activity.

Long‐term agricultural field experiments are essential for quantifying changes in soil properties and associated crop productivity that occur slowly but continue over long periods. Initiated in 1894 in the South of Denmark, the Askov long‐term experiment (LTE) is among the few LTEs in the world that continued for more than 125 years. The experiment compares different rates of nitrogen, phosphorus, and potassium applied in mineral fertilizers or animal manure with abundant treatment replicates. With well‐managed treatments, detailed yield records, and a soil archive dating back to 1923, today the Askov‐LTE represents a unique platform for research not envisioned at the start of experiments. Here, we provide a short description of site characteristics and experimental layout, examples of historic crop yields and changes in soil carbon (C) content. We provide short overviews of some of the studies that have used the Askov‐LTE as a research platform. These include crop yield‐related research; models simulating changes in soil C; availability of soil P reserves; indicators of biological, chemical, and physical soil quality; and studies related to prehistoric archaeology. Finally, we offer some reflections on long‐term field experimentation. [ABSTRACT FROM AUTHOR]