Tillage and rotational effects on exchangeable and enzyme-labile phosphorus forms in conventional and organic cropping systems

Authors: Dao, Thanh; Schomberg, Harry; Cavigelli, Michel

Submitted to: Nutrient Cycling in Agroecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2014
Publication Date: 12/12/2014
Citation: Dao, T.H., Schomberg, H.H., Cavigelli, M.A. 2014. Tillage and rotational effects on exchangeable and enzyme-labile phosphorus forms in conventional and organic cropping systems. Nutrient Cycling in Agroecosystems. 101:153-165.

Interpretive Summary: Phosphorus is a required nutrient for the normal development and growth of plants and supplemental phosphorus is needed in most cultivated soils. Animal manure is added to supply needed nutrients to promote the optimal production of grain crops and forages in traditional farming systems that have both a crop and a livestock component. ARS scientists investigated the cycling of phosphorus added to a conventional chisel-till, a no-till, and three organic crop management systems and compared the release of different forms of available phosphorus from each system to support crop production. Phosphorus was added in the form of commercial fertilizers in the conventional tillage systems or as poultry litter in the organic systems. Phosphorus is unevenly distributed with soil depth, with high levels in the soil surface and makes managing the accumulation of phosphorus in the surface of no-till soils and loose soil of conventionally-tilled soils a high priority in order to minimize the risk of loss to runoff. In the organic crop management systems, the annualized rate of phosphorus additions over the last 12 years of the study are conservatively within soil test recommendations for corn or soybean optimal yield. However, surpluses of phosphorus occurred in the organic-managed soils and were likely caused by low grain yields in dry years and poor weed control in some years. There were large accumulations of inorganic and organic forms of available phosphorus. The latter organic forms are not detected in routine soil testing procedures. The excess in available phosphorus not taken up by the crops accumulate in litter-amended soil, and can be lost, and potentially cause excessive algae growth and degrade water quality of estuaries such as the Chesapeake Bay. In time, a conservative replacement rate for phosphorus removed in crop harvests may help lower the rate of available phosphorus released from soil. However, past soil phosphorus build-up events have continued to contribute to the present-day high levels of soluble phosphorus twelve or more years later. The legacy of previously added phosphorus is long-lived and underscores the importance of more accurate estimates of nutrient requirements of crops when manure is the primary phosphorus source. As there are limited economically-viable sources of nutrients that can be used in organic farming, the need is citical for a more accurate methodology and tools for estimating nutrient supplying capacity of poultry litter, and animal manure in general to better determine actual need of crops and efficiently utilize manure nutrients in organic crop production.

Technical Abstract: The transformations of crop residues and bio-fertilizers used as primary sources of nutrients for organic grain and forage production are influenced by soil management practices. The effects of management of the near-surface zone on labile phosphorus (P) forms were studied in soil under three organic and two conventional crop management systems in the mid-Atlantic region of the U.S. after 17 years of continuous cultivation. The spatial variability and stratification in the distribution of extractable soil inorganic P (Pi), organic P forms, and total P in the 0-20 cm soil depth increment showed that managing surface accumulation of labile inorganic Pi (EEPi) and ligand-exchangeable enzyme-labile organic P (EDTA-PHP) remained a high priority in chisel-till (CT) and no-till (NT) management systems. In the organic crop management systems, P additions on an annualized basis may be within reasonable soil test recommendation during 2001-2013. Nonetheless, P surpluses in soil likely caused by low grain yields in dry years resulted in large pools of exchangeable EEPi and equally large pools of enzyme-labile EDTA-PHP. In addition, the difference in the P loading rates between the conventional and organic treatments suggested that overestimating plant P requirements contributed to soil P surpluses because routine soil testing procedures did not account for the presence and size of the soil enzyme-labile P pool. The impact of large P additions is long-lived as they continued to contribute to elevated soil total bioactive P concentrations 12 or more years later. Therefore, accurate estimates of crop P requirements and lability of P pools in soil, and real-time plant and soil sensing systems are critical considerations to optimally manage manure-derived nutrients and soil P status in organic crop management systems.