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Penn Vet Research Centers

Penn Vet's research centers are recognized throughout the nation and the world for groundbreaking advances in comparative oncology, health and productivity in food animal herds and flocks, infectious disease, regenerative medicine, and neuroscience.

Dou Laboratory

Nitrogen, Phosphorus, and Integrated Management

Nitrogen and phosphorus are essential nutrients for growing plants and animals. Insufficient nutrients diminish yields; excessive applications translate to wasted resources and environmental pollution. The work of Dr. Dou’s group features an integrated system nutrient management approach, coupling nutrient optimization in animal feeding with manure management and targeted nutrient application to crops for enhanced production efficiency and reduced environmental footprint.

  • Please see relevant projects and publications in 'Research Projects' and 'Publications' tab below.

Integrated whole farm management based on nutrients (pathogen) flow pathway

 

Whole Farm Management 

Pathogens, Antimicrobial Resistance (AMR), and Livestock Farming 

Livestock animals are important reservoirs of zoonotic pathogens as well as antimicrobial resistant determinants (antibiotic residues, AMR microbes, and AMR genes).  What happens to these “microbial pollutants” in the post-shed environment? How long do they survive under different management conditions? What is their distribution pattern in the intrinsically linked farming sectors and the dissemination pathways to the broader terrestrial and aquatic ecosystems? What intervention may help mitigate relevant risks associated with animal farming concerning food safety and public health? The research of Dr. Dou’s group addresses some of these critical issues.

  • Please see relevant projects and publications in 'Research Projects' and 'Publications' tab below.

Food Waste, Food Security, and Sustainability

Sustainable food security is an issue that intersects many of the contemporary challenges the world is struggling to deal with today, e.g. water scarcity, water pollution, resource limitation, land degradation, habitat and biodiversity loss, climate change, and hunger and poverty.  Dr. Dou collaborates with national and international experts to examine sustainable food security issues from multiple dimensions, such as food waste reduction and reuse, engaging, and empowering smallholder farmers, etc. 

  • Please see relevant projects and publications in 'Research Projects' and 'Publications' tab below.

Developing Novel Feeds from Food Waste and Crop Residue Biomass from Cows

Livestock farming is at a crossroads, its sustainable future challenged by competing interests for limited resources and urgent need to mitigate environmental and climate footprints, amid a rapidly growing global demand for animal protein. One viable solution overlooked, is to leverage the innate ability of animals as nature’s most effective recyclers, able to utilize a wide variety of plant biomass materials as feed resources in producing meats, milk, and eggs. This project will deploy an innovative sequential fermentation approach to create novel feeds for dairy cows using food waste of fruit and vegetable discards and post-harvest crop residues such as wheat straw and spent mushroom substrates, abundantly available but currently wasted or landfilled. 

  • Please see relevant projects and publications in 'Research Projects' and 'Publications' tab below.

Contact Information

For more information, please contact:

Dr. Zhengxia Dou, Penn Vet

Dr. Zhengxia Dou
Email: douzheng@vet.upenn.edu

Dou Laboratory
Dr. Zhengzia Dou Zhengxia Dou, PhD, Professor
John Toth John D. Toth, Research Specialist
Dr. Ting Chen, Visiting Professor
Ms. Yunyun Li, Visiting PhD Candidate
Past Members
Dr. Youwen You Post-doctoral Fellow
Dr. Fanghao Wang Post-doctoral Fellow
Dr. Jude Fiorini Post-doctoral Fellow
Dr. Leidy Chapuis-Lardy Post-doctoral Fellow
Dr. Chong Wang Visiting Scholar
Dr. Gangya Zhang Visiting Scientist
Dr. Zimin Wei Visiting Scholar
Dr. Weifeng Zhang
 Visiting Scholar
Dr. Donghui Zhang  Visiting Scholar

 PSNT (pre-sideddress soil nitrate test)

A widely evaluated and reliable soil test to assess if a corn crop will or will not benefit from sideddress nitrogen fertilization, potentially saving fertilizer cost for the producer and reducing nutrient losses to the environment (a win-win situation).

Contact:

John Toth
Phone: 610-925-6327
Email: jdtoth@vet.upenn.edu 

Dairy farm nutrient efficiency evaluation

Dr. Dou and her colleagues at the Center for Animal Health and Productivity offer a number of management tools and interventions to help dairy producers optimize nutrient flow with enhanced animal performance, improved farm profitability, and reduced environmental footprint. 

  • Dietary evaluation and ration formulation
  • MUN, a quick way to assess if overfeeding protein
  • Manure phosphorus, an easy test to see if overfeeding phosphorus
  • Nutrient use efficiency in a nutshell
  • Reproduction, heifers for replacement, record analyses

Contacts

ldbaker@vet.upenn.edu
douzheng@vet.upenn.edu
galligan@vet.upenn.edu
ferguson@vet.upenn.edu

 Courses/Lectures

  • Animal production systems (a 2-credit elective course)
  • Nutrition and pollution (lecture)
  • Manure-borne pathogens and the environment fate (lectures)
  • Soils and global food security (lectures)
  • Food wastage along the food supply chain (lectures)

Scope of Research

Recently funded research project: Developing novel feeds via bioprocessing of food waste and crop residue biomass to support sustainable dairy production

USDA NIFA program: IDEAS (Inter-Disciplinary Engagement in Animal Systems)

Project duration: 2/1/2022 – 1/31/2025

Award #: 2022-68014-36664; CRIS #: 1028184 

Project key personnel:  

  •  Penn Vet: Dr. Linda Baker, Dr. Dipti Pitta, Dr. Joe Bender, Dr. Brianna Parsons
  • University of Minnesota: Dr. Bo Hu, Dr. Gerald Shurson
  • Drexel University: Dr. Jonathan Deutsch

Project summary: Livestock farming is at a crossroads, its sustainable future challenged by competing interests for limited resources and urgent need to mitigate environmental and climate footprints, amid a rapidly growing global demand for animal protein.

One viable solution overlooked, is to leverage the innate ability of animals as nature’s most effective recyclers, able to utilize a wide variety of plant biomass materials as feed resources in producing meats, milk, and eggs.

This project will deploy an innovative sequential fermentation approach to create novel feeds for dairy cows using food waste of fruit and vegetable discards and post-harvest crop residues such as wheat straw and spent mushroom substrates, abundantly available but currently wasted or landfilled.

First, treatment of crop residue biomass will proceed with selected fungal strains able to alter lignin-cellulose complexes through aerobic fermentation to enhance nutrient availability, then combining fruit and vegetable wastes for bacteria-driven anaerobic fermentation to create safe nutritionally enhanced novel feeds (Objective 1).

Fermented novel feeds will be incorporated into cow diets; milk production, rumen microbiota and enteric methane emissions determined (Objective 2).

Broader impacts resulting from novel feeds partially substituting conventional feed ingredients, in terms of resources spared and greenhouse gas emissions avoided, will be quantitatively assessed (Objective 3).

Interwoven with interdisciplinary research are educational activities with classroom learning, research immersion, and hands-on problem-solving and opportunity-finding on selected dairy farms (Objective 4).

This project will generate groundbreaking scientific information to advance technological innovation, and provide evidence-based practical solutions for reducing farming costs and improving agri-food system efficiency. This proposal addresses the priority topics of “environmental synergies of animal production” for reduced “emissions to the atmosphere and hydrosphere” under Program Area Priority A1261 “Inter-Disciplinary Engagement in Animal Systems (IDEAS)”.

Previous Projects

  • Food waste reduction and reuse, food security, sustainability: (i) The Last Food Mile conference, USDA AFRI, 2014. (ii) Household food discard – sample collection and characterization, The University Research Foundation. 2015. (iii)  ReFeed – returning wasted food to the human food chain via animal feeding for enhanced food-energy-water and environmental sustainability. NSF-NSFC. PI. Pending.
  • Mitigating antibiotic resistance risks through a holistic approach. Pennsylvania Department of Agriculture. PI. 2017.
  • Antibiotic resistance genes in the fecal microbiome of dairy cattle and humans. PennCHOP. PI. 2017-18.
  • Increasing Economic and Environmental Sustainability of Small and Medium 
    Sized Dairy Farms.  USDA AFRI. Co-PI. 2009-2013.
  • Integrated management program on lower Susquehanna dairy farms.  National Fish and Wildlife Foundation, PI. 2008-2012.
  • Survival and Transport of Escherichia coli O157 and Salmonella Newport in Manure and Manured Soils. USDA National Research Initiative (NRI) Competitive Grants Program, PI. 2007-2012.
  • Enhancing Veterinary School Faculty and Student Competence in a Global Economy through Curriculum Building and Strengthening Collaboration with Chinese Dairy Industry.   USDA International Science and Education (ISE) Competitive Grants program, Co-PI. 2008-2011.
  • Development and dissemination of optimal phosphorus management technologies on dairy farms. USDA Initiative for Future Agriculture and Food Systems(IFAFS) competitive program, PI. 2001-2006.
  • Assessing the impact on surface and ground waters of N- vs. P based manure application strategies. USDA, National Research Initiative (NRI) Competitive Research Program, PI. 2001-2005.
  • Using chemical amendments to fix phosphorus and nitrogen in animal manure to reduce potential losses. US EPA Chesapeake Bay Program, PI. 2001-2002.
  • Coal fly ash as bedding amendment for restrained bacterial growth and reduced nutrient losses. Pennsylvania Animal Health Commission, PI. 2001-2002.Characterizing manure phosphorus to identify management strategies for reduced water pollution. Pennsylvania Department of Agriculture. PI. 2000.
  • Development of a farm model to reduce nitrogen losses in dairy cows and improve farm economics. USDA CSREES. Co-PI. 1997-2004.

 Nutrient management

  • Li, T, W. Zhang, D. Powlson, X. Liu, X. Chen, F. Zhang,  Z. Dou.  Enhanced-efficiency fertilizers are not a panacea for resolving the nitrogen problem.  Global Change Biology. doi: 10.1111/gcb.13918 2017
  • Cade-Menuu, B., Z. He, and Z. Dou. Comparison of phosphorus forms in three extracts of dairy feces by solution P-31 NMR analysis. Communications in Soil Science and Plant Analysis. 22 Jun 2015. DOI:10.1080/00103624.2015.1047512
  • Yan, Z., Q. Chen, Z. Dou, J. Alvas. Phosphorus in China's intensive vegetable production systems: Over-fertilization, soil enrichment, and environmental implications. J. Environ. Qual. 42:982-989, 2013.
  • Kaitlyn Lutz, D. Remsberg, Z. Dou. Nutrient Mass Balance and Environmental Implications on Two Pennsylvania Dairy Farms. The Bovine Practitioner, summer, 2011.
  • Wang, F.H., L. Ma, W.Q. Ma, F.S. Zhang, Z. Dou. J.T. Sims. The phosphorus footprint of China's food chain: implications for food security, natural resource management, and environmental quality. J. Environ. Qual. 40:1081-1089, 2011.
  • Remsburg, D., RJ. Munson, RD Elliot, LD. Baker, JD. Ferguson, DT. Galligan, Z. Wu, CF. Ramberg, and Z. Dou. Case Report – Two nutritional interventions in dairy cattle herds helped improve productivity and animal health while reducing the environmental footprint.  The Bovine Practitioner, Vol. 44, No.2:2010.
  • Wang, F.H., Z. Dou, L. Ma, W.Q. Ma, F.S. Zhang. Nitrogen Mass Flow in China’s Animal Production System and Environmental Implications. J. Environ. Qual. 39:1537-1544. 2010.
  • He, Z., H. Zhang, G. Toor, Z. Dou, W. Honeycutt, B. Haggard, and M. Reiter. 2010.  Phosphorus distribution in sequentially-extracted fractions of biosolids, poultry litter and granulated products. Soil Sci. 175:154-161. 2010.
  • Dou, Z., C. Ramberg, J.D. Toth, J. Ferguson, R. Kohn, K. Knowlton, L. Chase, Z. Wu. A fecal test for assessing P overfeeding: Evaluation using an extensive farm dataset. J. Dairy Sci. 93:830-839. 2010
  • He, Z. and Z. Dou. Phosphorus forms in animal manure and the impact on soil P status. In: Dellaguardia CS (ed) Manure: management, use and environmental impacts. Nova Science Publishers, Inc., New York, N.Y., pp 83-114, 2010.
  • He, Z., C.W. Honeycutt, T.S. Griffin, B.J. Cade-Menun, P.J. Pellechia, and Z. Dou. Phosphorus forms in conventional and organic dairy manure identified by solution and solid state P-31 NMR spectroscopy.  J. Environ. Qual. 38:1909-1918. 2009.
  • Dou, Z., C.R. Chen, C. F. Ramberg, J.D. Toth, Y. Wang, A.N. Sharpley, S.E. Boyd, D. Williams, and Z.H. Hu. Phosphorus speciation and sorption-desorption characterisitcs in heavily manured soils. Soil Sci. Soc. Am. J. 73:93-101, 2009.
  • Cui, Z., X. Chen, Z. Dou, F. Li, Q. Sun, F. Zhang, J. Li, Y. Ye, Z. Yang, Q. Zhang, and C. Liu. Regional evaluation of critical nitrogen concentrations in winter wheat production of the North China Plain. Agron. J. 101:159-166, 2009.  
  • McDowell, R., Z. Dou, J.D. Toth, B. Cade-Menun, P. Kleiman, K. Soder, L. Soporito. Extractability and speciation of phosphorus in feeds and feces of different dairy herds.  J. Environ. Qual. 37:741-752, 2008.
  • Kleinman, P., D. Sullivan, A. Wolf, R. Brandt, Z. Dou, H. Elliott, J. Kovar, A. Leytem, R. Maguire, P. Moore, L. Saporito, A. Sharpley, A. Shober, T. Sims, J. Toth, G. Toor, H. Zhang, T. Zhang. Selection of a water-extractable phosphorus test for manures and biosolids as an indicator of runoff loss potential. J. Environ. Qual. 36:1357–1367, 2007.
  • Dou, Z., C. F. Ramberg, L. Chapuis-Lardy, J. Fiorini, J.D. Toth, J.D. Ferguson. A novel approach for measuring and managing potential phosphorus loss from dairy cattle feces. Environ Sci. Technol. 41:4361-4366, 2007.
  • Toth, J.D., Z. Dou, J.D. Ferguson, and C.F. Ramberg, Jr. Nitrogen- vs. phosphorus-based dairy manure applications to field crops: Nitrate and phosphorus leaching and soil phosphorus accumulation.  J. Environ. Qual. 35: 2302-2312, 2006.
  • Maguire, R.O., Z. Dou, B.C. Joern, J.T. Sims, and J. Brake.  Manipulating dietary phosphorus to decrease environmental impacts of animal agriculture. J. Environ. Qual. 34:2093-2103, 2005.
  • Toor, G., J.T. Sims, and Z. Dou*. Reducing phosphorus in dairy diets improves farm nutrient balances and decreases the risk of nonpoint pollution of surface and ground waters. Agric. Ecosys. Environ. 105:401-411, 2005.
  • Zhang, G., Z. Dou, J.D. Toth, J. Ferguson. Use of flyash as environmental and agronomic amendments. Environ Geochem Health. 26:129-134, 2004.
  • Chapuis-Lardy, L.,J. Fiorini, J.D. Toth, J.D. Ferguson, and Z. Dou. Phosphorus concentrations in dairy feces: variability and affecting factors. J. Dairy Sci. 87:4334-4341, 2004.
  • Dou, Z., J.D. Ferguson, J. Fiorini, J.D. Toth, L.E. Chase, K.F. Knowlton, R.A. Kohn, J.T. Sims, and Z. Wu. Phosphorus feeding levels and critical control points on dairy farms. J. Dairy Sci. 86:3787-3795, 2003.
  • Dou, Z., G. Zhang, J.D. Toth, W. Stout, and J.D. Ferguson. Efficacy of alum and coal combustion byproducts in stabilizing manure phosphorus.  J. Environ. Qual. 32:1490-1497, 2003.
  • Dou, Z., K.F. Knowlton, R.A. Kohn, L.D. Satter, Z. Wu, G. Zhang, J.D. Toth, and J.D. Ferguson. Phosphorus characteristics in dairy feces affected by diets. J. Environ. Qual. 31:2058-2065, 2002.
  • Boston, R., Z. Dou, and W. Chalupa. Nutrition – ration formulation: (d) Models in nutritional management.  In: Encyclopedia of Dairy Science. Academic Press. London, UK. P. 2378-2389, 2002.
  • Dou, Z.,  D. T. Galligan, R.D. Allshouse, J.D. Toth, C.F. Ramberg, and J.D. Ferguson. Manure Sampling for Nutrient Analysis: Variability and Sampling Efficacy. J. Environ. Qual. 30:1432-1437, 2001.
  • Dou, Z., D. T. Galligan, C.F. Ramberg, Jr., C. Meadows, and J.D. Ferguson. A survey of dairy farming in Pennsylvania: nutrient management practices and implications. J. Dairy Sci. 84:966-973, 2001.
  • Dou, Z., J.D. Toth, D.T. Galligan, C.F. Ramberg, Jr., and J.D. Ferguson. Laboratory procedures for characterizing manure phosphorus. J. Environ. Qual. 29: 508-514, 2000.
  • Dou, Z., L.E. Lanyon, J.D. Ferguson, R.A. Kohn, R.C. Boston, and W. Chalupa. An integrated approach for managing nitrogen on dairy farms: Evaluating farm performance using the dairy nitrogen planner.  Agron. J.  90:573-581, 1998.
  • Kohn, R.A., Z. Dou, J.D. Ferguson, and R.C. Boston. A sensitivity analysis of nitrogen losses from dairy farms. J. Environ. Manag. 50: 417-428, 1997.
  • Dou, Z., R.A. Kohn, J.D. Ferguson, R.C. Boston, W. Chalupa, and J.D. Newbold. Managing nitrogen on dairy farms: An integrated approach.  J. Dairy Sci. 79:2071-2080, 1996.
  • Dou, Z., J.D. Toth, J.D. Jabro, R.H. Fox, and D.D. Fritton. Soil nitrogen mineralization during laboratory incubation: dynamics and model fitting.  Soil Biol. Biochem. 28: 625-632, 1996.
  • Dou, Z., R.H. Fox, and J.D. Toth. Seasonal soil nitrate dynamics in corn as affected by tillage and N source. Soil Sci. Soc. Am. J. 59: 858-864, 1995.
  • Jabro, J.D., J.D. Toth, Z. Dou, R.H. Fox, and D.D. Fritton.  Evaluation of nitrogen  version of LEACHM for predicting nitrate leaching. Soil Sci. Vol. 169, no. 3. 209-217, 1995.
  • Dou, Z. and R. H. Fox. Using NCSWAP to simulate seasonal nitrogen dynamics in soil and corn. Plant Soil. 177: 235-247, 1995.
  • Dou, Z. and R.H. Fox. The contribution of nitrogen from legume cover crops double-cropped with winter wheat to tilled and non-tilled maize. Eur. J. Agron. 3: 93-100, 1994.
  • Dou, Z., R.H. Fox, and J.D. Toth.  Tillage effect on seasonal nitrogen availability in corn supplied with legume green manures. Plant Soil. 162: 203-210, 1994.

 

Pathogens, antibiotics, and antimicrobial resistance

  • Cao, J. C. Wang, Z. Dou, M. Liu, D. Ji. Hyphospheric impacts of earthworms and aubuscular mycorrhizal fungus on soil bacterial community to promote oxytetracycline degradation. J. Hazardous Materials. 341:346-354. 2018.
  • Pitta, D., Z. Dou, S. Kumar, N. Indugu, J. Toth, B. Vecchiarelli, and B. Bhukya.   Metagenomic evidence of the prevalence and distribution patterns of antimicrobial resistance genes in dairy agroecosystems.  Foodborne Pathogens
    and Disease. DOI: 10.1089/fpd.2015.2092, 2016.
  • Cao, J., C. Wang, Z. Dou, and D. Ji. Independent and combined effects of oxytetracycline and antibiotic-resistant E. coli O157:H7 on soil microbial activity and partial nitrification processes. Soil Biol. Biochem. 98:138-147. DOI:10.1016/j.soilbio.2016.03.014. 2016.
  • Toth, J.D., H.W. Aceto, S.C. Rankin, J.D. Vanderhoef, and Z. Dou.  Survey of animal-borne pathogens in the farm environment of 13 dairy operations. J. Dairy Sci., 96: 5756-5761, 2013.
  • Toth, John, Helen W. Aceto, Shelley C. Rankin, Chitrita DebRoy, and Z. Dou. Accelerating the Deactivation of Salmonella enterica Serovar Newport and Escherichia coli 0157:H7 in Dairy Manure by Modifying pH or Temperature. Open Waste Manage. J. 5:11-18. 2012.
  • Toth, J.D., S. Rankin, H. Aceto, Z. Dou. Survival characteristics of Salmonella enterica serovar Newport in the dairy farm environment.  J. Dairy Sci. Vol. 94:5238-5246. 2011.
  • Toth, J.D., Y. Feng, Z. Dou. Veterinary Antibiotics at Environmentally Relevant Concentrations Inhibit Soil Iron Reduction and Nitrification. Soil Biol Biochem Vol. 43:2470-2472, 2011.
  • Kristula, M.A., Z. Dou, J.D. Toth, B. Smith, N. Harvey, and M. Saho. Evaluation of free stall mattress bedding treatments to reduce mastitis bacterial growth. J. Dairy Sci. 91:1885-1892, 2008.
  • You, Y., S. Rankin, H. Aceto, C. Benson, and Z. Dou. Fate of Salmonella Newport in manure and manured-soil. Appl. Environ. Microbiol. 72:5777-5783, 2006.

 

Food security and sustainability

  • Cui, Z., H. Zhang….20 others…, F. Zhang, and Z. Dou. Pursuing sustainable productivity with millions of smallholder farmers. Nature. In press.
  • Dou, Z., J.D. Toth, M. Westendorf. Food waste for livestock feeding: Feasibility, safety, and sustainability implications. Global Food Security, https://doi.org/10.1016/j.gfs.2017.12.003. 2018.
  • Hou Jun, Weifeng Zhang 1,*, Pei Wang 1, Zhengxia Dou 2, Liwei Gao 3 and David Styles. Greenhouse gas mitigation of rural household biogas systems in China: A life cycle assessment. Energies. doi:10.3390/en10020239  2017.
  • Tong, Y., J. Liu, X. Li, J. Sun, A. Herzberger, W. Zhang, Z. Dou, F. Zhang. Cropping system conversion led to organic carbon change in China’s mollisols regions. Scientific Reports, doi:10.1038/s41598-017-18270-5  2017.
  • Zhang, W., G. Cao, X. Li, H. Zhang, C. Wang, Q. Liu, X. Chen, Z. Cui, J. Shen, R. Jiang, G. Mi, Y. Miao, F. Zhang, and Z. Dou. Closing yield gaps in China by empowering smallholder farmers. Nature. Doi:10.1038/nature19368. 2016.
  • Dou, Z., J. Ferguson, D. Galligan, A. Kelly, S. Finn, R. Giegengack.  Assessing food loss across U.S. supply chain and opportunities for reduction. Global Food Security. Volume 8, March 2016, Pages 19-26. doi:10.1016/j.gfs.2016.02.001, 2016.
  • Galligan, D., JD Ferguson, A Kelly, Z Dou, Z Wu. Aspects of Animal Production in Global Food Supply in Globalization: The Crucial Phase. B Spooner (ed).  University of Pennsylvania. 2014.
  • Li, Y., W. Zhang, L. Ma, L. Wu, J. Shen, W.J. Davies, O. Oenema, F. Zhang, and Z. Dou. An analysis of China's grain production: looking back and looking forward.  Food and Energy Security. DOI: 10.1002/fes3.41. 2013.
  • Sims, T. L. Ma, F. Zhang, Z. Dou, O. Onema. Nutrient Management, Food Security and the Environment: Global Challenges. J. Environ. Qual. 42:947-950, 2013.
  • Li, Y., W. Zhang, F. Zhang, O. Onema, and Z. Dou. An Analysis of China's fertilizer policies and its impacts on the industry and the environment. J. Environ. Qual. 42:972-981, 2013.
  • Zhang, W., Z. Dou*, P. He, X. Ju, D. Powlson, D. Chadwick, D. Norse, Y.Lu, Y. Zhang, L.Wu, X. Chen, F. Zhang.  New technologies reduce greenhouse gas emissions from nitrogenous fertilizer in China. Proc Natl Acad Sci USA, 10.1073/pnas.1210447110, Special Feature, Early Edition. 2013.
  • Li, H., Z. Li, Z. Dou, J. Zhang, and C. Wang. Earthworm (Aporrectae trapezoids)–mycorrhiza (Glomus intraradices) interaction and nitrogen and phosphorus uptake by maize. Biol. Fert. Soils. Vol. 48:75-85. 2012.
  • Ju, X., C. Kou, P. Christie, Z. Dou, F. Zhang. Changes in the soil environment from excessive application of fertilizers and manures to two contrasting intensive cropping systems on the North China Plain. Environmental Pollution. 145 (2):497-506, 2007.