Jianbo Shen

Phosphorus Dynamics: From Soil to Plant

With increasing demand of agricultural production and as the peak in global production will occur in the next decades, phosphorus (P) is receiving more attention as a nonrenewable resource ([Cordell et al., 2009][1]; [Gilbert, 2009][2]). One unique characteristic of P is its low availability due to

Improving crop productivity and resource use efficiency to ensure food security and environmental quality in China

In recent years, agricultural growth in China has accelerated remarkably, but most of this growth has been driven by increased yield per unit area rather than by expansion of the cultivated area. Looking towards 2030, to meet the demand for grain and to feed a growing population on the available arable land, it is suggested that annual crop production should be increased to around 580 Mt and that yield should increase by at least 2% annually. Crop production will become more difficult with climate change, resource scarcity (e.g. land, water, energy, and nutrients) and environmental degradation (e.g. declining soil quality, increased greenhouse gas emissions, and surface water eutrophication). To pursue the fastest and most practical route to improved yield, the near-term strategy is application and extension of existing agricultural technologies. This would lead to substantial improvement in crop and soil management practices, which are currently suboptimal. Two pivotal components are required if we are to follow new trajectories. First, the disciplines of soil management and agronomy need to be given increased emphasis in research and teaching, as part of a grand food security challenge. Second, continued genetic improvement in crop varieties will be vital. However, our view is that the biggest gains from improved technology will come most immediately from combinations of improved crops and improved agronomical practices. The objectives of this paper are to summarize the historical trend of crop production in China and to examine the main constraints to the further increase of crop productivity. The paper provides a perspective on the challenge faced by science and technology in agriculture which must be met both in terms of increased crop productivity but also in increased resource use efficiency and the protection of environmental quality.

P for Two, Sharing a Scarce Resource: Soil Phosphorus Acquisition in the Rhizosphere of Intercropped Species

Phosphorus (P) scarcity and the need for ecologically sound intensification of agroecosystems are major challenges we face. To improve nutrient efficiency in agriculture, especially for P, multispecies crop stands may outperform their monospecific counterparts, especially under low input conditions. There is increasing evidence that biomass, grain yield and nutrient acquisition are improved in cereal/legume intercropping systems, relative to cereal or legume grown alone. Hereafter, we consider these observations, outline the underlying mechanisms, and examine recent work that advances our knowledge of how cereal/legume intercropping systems acquire P in their rhizospheres, through various types of positive belowground-interactions. First, we discuss how complementarity may operate when cereals are intercropped with legumes by addressing cases of complementary use of soil P resources in space and time, and showing how functionally diverse intercropped species can use different pools of soil P. Then we address examples of facilitation, i.e. positive interactions between two intercropped species, in which the legume (or cereal) may increase P availability for the benefit of the intercropped cereal (or legume). Finally, the relevance of a range of root-induced or microbially-mediated rhizosphere processes driving P acquisition are discussed.

Role of phosphorus nutrition in development of cluster roots and release of carboxylates in soil-grown Lupinus albus

The present study examined the effect of phosphorus (P) limitation on cluster root formation and exudation of carboxylates by N2-fixing white lupin (Lupinus albus L. cv. Kiev) grown in a P-deficient sandy soil. Plants received 10 (limited P) or 200 μg P g−1 soil as FePO4 (adequate P) and were grown in a phytotron at 20/12 °C (12/12 h) for 76 days in soil columns. Cluster root formation was assessed and root exudates were collected at 9-day intervals. Shoot and root dry weights were higher in plants grown in the adequate-P compared to the limited-P treatment for 67 days. No clear difference in the total root length was observed between two P treatments before day 58. However, the specific root length increased rapidly from 17 m g−1 DW at day 40 to 28 m g−1 at day 49 in the P-limited plants, but decreased in the P-adequate plants. The effect of P limitation on enhancement of cluster root formation was observed from day 40 and reached the maximum at day 58. The number of cluster roots was negatively correlated with the P concentration in both roots and shoots. Phosphorus limitation increased exudation of citrate from day 40. The exudation of citrate displayed a cyclic pattern throughout the experiment, and appeared related to internal P concentration in plants, particularly P concentration in shoots. The sorption of exogenously added citrate in the soil was also examined. The amount of extractable citrate remained unchanged for 2 h, but decreased thereafter, suggesting that the soil had a low capacity to sorb citrate, and the rate of its decomposition by microorganisms was slow. Collecting solution leached through a soil column is a simple and reliable method to acquire root exudates from white lupin grown in soil. The results suggest that formation of cluster roots and exudation of citrate in white lupin are regulated by P concentration in shoots.

Sustainable Phosphorus Management and the Need for a Long-Term Perspective: The Legacy Hypothesis

Perspective: The Legacy Hypothesis Philip M. Haygarth,*,† Helen P. Jarvie,‡ Steve M. Powers, Andrew N. Sharpley, James J. Elser, Jianbo Shen, Heidi M. Peterson, Neng-Iong Chan, Nicholas J. K. Howden, Tim Burt, Fred Worrall, Fusuo Zhang, and Xuejun Liu †Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, U.K. ‡Centre for Ecology and Hydrology, OX10 8BB, Wallingford, Oxfordshire, U.K. University of Notre Dame, Environmental Change Initiative, South Bend, Indiana 46617, United States Division of Agriculture, University of Arkansas, Fayetteville, Arkansas 72701, United States School of Life Sciences, Arizona State University, Tempe, Arizona 85287, United States Center for Resources, Environment and Food Security, China Agricultural University, Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing 100193, P. R. China Minnesota Department of Agriculture, Pesticides and Fertilizer Management Division, Saint Paul, Minnesota 55155, United States University of Bristol, Queen’s School of Engineering, BS8 1TH, Bristol, U.K. Durham University, Department of Geography, DH1 3LE, Durham, U.K. Durham University, Department of Earth Sciences, DH1 3LE, Durham, U.K.

Closing yield gaps in China by empowering smallholder farmers

Sustainably feeding the world’s growing population is a challenge, and closing yield gaps (that is, differences between farmers’ yields and what are attainable for a given region) is a vital strategy to address this challenge. The magnitude of yield gaps is particularly large in developing countries where smallholder farming dominates the agricultural landscape. Many factors and constraints interact to limit yields, and progress in problem-solving to bring about changes at the ground level is rare. Here we present an innovative approach for enabling smallholders to achieve yield and economic gains sustainably via the Science and Technology Backyard (STB) platform. STB involves agricultural scientists living in villages among farmers, advancing participatory innovation and technology transfer, and garnering public and private support. We identified multifaceted yield-limiting factors involving agronomic, infrastructural, and socioeconomic conditions. When these limitations and farmers’ concerns were addressed, the farmers adopted recommended management practices, thereby improving production outcomes. In one region in China, the five-year average yield increased from 67.9% of the attainable level to 97.0% among 71 leading farmers, and from 62.8% to 79.6% countywide (93,074 households); this was accompanied by resource and economic benefits.

Integrating legacy soil phosphorus into sustainable nutrient management strategies for future food, bioenergy and water security

Legacy phosphorus (P) that has accumulated in soils from past inputs of fertilizers and manures is a large secondary global source of P that could substitute manufactured fertilizers, help preserve critical reserves of finite phosphate rock to ensure future food and bioenergy supply, and gradually improve water quality. We explore the issues and management options to better utilize legacy soil P and conclude that it represents a valuable and largely accessible P resource. The future value and period over which legacy soil P can be accessed depends on the amount present and its distribution, its availability to crops and rates of drawdown determined by the cropping system. Full exploitation of legacy P requires a transition to a more holistic system approach to nutrient management based on technological advances in precision farming, plant breeding and microbial engineering together with a greater reliance on recovered and recycled P. We propose the term ‘agro-engineering’ to encompass this integrated approach. Smaller targeted applications of fertilizer P may still be needed to optimize crop yields where legacy soil P cannot fully meet crop demands. Farm profitability margins, the need to recycle animal manures and the extent of local eutrophication problems will dictate when, where and how quickly legacy P is best exploited. Based on our analysis, we outline the stages and drivers in a transition to the full utilization of legacy soil P as part of more sustainable regional and global nutrient management.

Is there a critical level of shoot phosphorus concentration for cluster-root formation in Lupinus albus?

This study examined the effects of localised phosphorus (P) supply on cluster-root formation and citrate exudation in white lupin (Lupinus albus L. cv. Kiev Mutant). White lupin plants were grown in nutrient solutions with a range of P supplies in a split-root system with one root half deprived of P and the other root supplied with 0, 2, 5, 8, 10 or 75 μm P. Plants were also grown in soil with or without organic matter added to the top layer. The proportion of cluster roots as a percentage of the total root biomass decreased similarly on both root halves with increasing P supply in the hydroponic experiments. More than 18% of the P taken up by the P-supplied root halves was incorporated into the P-deprived halves. Irrespective of the P supply or organic matter addition in the experiments, the proportion of cluster roots and the rate of citrate exudation decreased sharply with increasing P concentration in the shoots up to a critical level of 2-3 mg P g-1 dry weight. In contrast, the rate of proton release was higher in P-deprived root halves than in P-supplied ones. The formation of cluster roots is regulated by shoot P concentration with a critical level of 2-3 mg g-1. Citrate exudation is predominantly governed by shoot P status, whereas proton release strongly responds to local P supply.

Update on White Lupin Cluster Root Acclimation to Phosphorus Deficiency Update on Lupin Cluster Roots

Phosphorus (P) is one of 17 essential elements (nutrients) required for plant growth (Tiessen, 2008; Cordell et al., 2009). Although bound P is quite abundant in many soils, it is largely unavailable for uptake. As such, P is frequently the most limiting element for plant growth and development. Crop yield on 40% to 60% of the world’s arable land is limited by P availability. Mined rock phosphate is the primary source of P fertilizer. Approximately 90% of all mined rock phosphate is used for agriculture (Tiessen, 2008; Cordell et al., 2009). However, rock phosphate is a nonrenewable resource (Steen, 1998; Cordell et al., 2009), and easily mined, high-quality rock phosphate sources are projected to be depleted within 30 to 50 years (Steen, 1998; Tiessen, 2008; Cordell et al., 2009). Peak P production is projected to occur in 2035 to 2040 (Cordell et al., 2009). In addition, the world’s major reserves of rock phosphate are located in geographical areas where uncertain political issues could limit access to the world’s P resources. Sustainable management of P in agriculture requires that plant biologists discover mechanisms that enhance P acquisition and exploit these adaptations to make plants more efficient at acquiring P, develop P-efficient germplasm, and advance crop management schemes that increase soil P availability. Cluster roots (Fig. 1), extremely specialized tertiary lateral root structures, are an important adaptive strategy of plants to cope with nutrient-poor, P-depleted soils (Dinkelaker et al., 1995; Neumann and Martinoia, 2002; Vance et al., 2003; Lambers et al., 2006). They are produced on plants from a diverse range of families (Dinkelaker et al., 1995; Watt and Evans, 1999; Shane and Lambers, 2005). White lupin (Lupinus albus) forms cluster roots in response to P starvation. Cluster roots are characterized as concentrated zones of tertiary lateral roots emerging in waves from secondary roots. Root hair density appears to be greater in mature cluster root zones than typical lateral roots. Such an adaptation leads to a striking increase in root surface area available for P uptake from the rhizosphere (Keerthisinghe et al., 1998; Neumann et al., 1999). Cluster root development and function involve a highly synchronous series of molecular and biochemical processes, including highly enhanced lateral root initiation, increased root hair formation, root exudation of organic acid chelators (citrate and malate), modified carbon assimilation, release of enzymes (acid phosphatase, ferric chelate reductases) into the rhizosphere, and more efficient uptake of P from the rhizosphere (Dinkelaker et al., 1989; Neumann et al., 1999; Watt and Evans, 1999; Liu et al., 2001, 2005; Miller et al., 2001; Uhde-Stone et al., 2003a, 2005; Wasaki et al., 2003). Advances have recently been made in understanding the molecular and biochemical events surrounding cluster root formation and function. As a crop, white lupin is a practical alternative to evaluate acclimation to P deficiency, particularly as related to cluster-rooted species (Johnson et al., 1996; Keerthisinghe et al., 1998; Watt and Evans, 1999; Neumann and Martinoia, 2002). Figure 1. White lupin P deficiency cluster roots emerge as waves of tertiary lateral roots along the axis of secondary roots.Department of Plant Nutrition, China Agricultural University, Key Laboratory of Plant-Soil Interactions, Beijing 100193, People’s Republic of China (L.C., J.S.); Department of Agronomy and Plant Genetics (L.C., B.B., C.P.V.) and Department of Soil, Water, and Climate (D.A.), University of Minnesota, St. Paul, Minnesota 55108; and United States Department of Agriculture Agricultural Research Service, St. Paul, Minnesota 55108 (B.B., C.P.V.)

How do roots elongate in a structured soil

In this review, we examine how roots penetrate a structured soil. We first examine the relationship between soil water status and its mechanical strength, as well as the ability of the soil to supply water to the root. We identify these as critical soil factors, because it is primarily in drying soil that mechanical constraints limit root elongation. Water supply to the root is important because root water status affects growth pressures and root stiffness. To simplify the bewildering complexity of soil-root interactions, the discussion is focused around the special cases of root elongation in soil with pores much smaller than the root diameter and the penetration of roots at interfaces within the soil. While it is often assumed that the former case is well understood, many unanswered questions remain. While low soil-root friction is often viewed as a trait conferring better penetration of strong soils, it may also increase the axial pressure on the root tip and in so doing reduce the rate of cell division and/or expansion. The precise trade-off between various root traits involved in root elongation in homogeneous soil remains to be determined. There is consensus that the most important factors determining root penetration at an interface are the angle at which the root attempts to penetrate the soil, root stiffness, and the strength of the soil to be penetrated. The effect of growth angle on root penetration implicates gravitropic responses in improved root penetration ability. Although there is no work that has explored the effect of the strength of the gravitropic responses on penetration of hard layers, we attempt to outline possible interactions. Impacts of soil drying and strength on phytohormone concentrations in roots, and consequent root-to-shoot signalling, are also considered.