Chemical characterization of soil phosphorus and organic matter in different cropping systems in Maine, U.S.A.

To minimize purchased inputs, sustainable agriculture systems often use green- or animal-manure as a nitrogen source for crops. The addition of these materials also have positive impacts on soil phosphorus (P) availability. This study was conducted to investigate the effects of animal- and legume-based cropping systems in place for 5-13 years on the chemistry of organic matter and soil P. Two cropping systems, Potato Ecosystem (ECO) located in Presque Isle, ME, U.S.A. and Grass Fertility (GF) located in Stillwater, ME, U.S.A. received animal manure. Two others, Liebman E Rotation (LER) in Stillwater, ME, U.S.A. and Porter Rotation (PR) in Presque Isle, ME, U.S.A. were primarily legume-based rotations. Estimated difference in carbon input (ha−1 year−1) between the amended and control plots were: ECO, 8100 kg C; GF, 1010 kg; LER, 950 kg C; and PR, 415 kg C. The dissolved organic carbon (DOC) concentration was higher (P < 0.10) in the treatment that received animal manure than those which only received legume residue. The E2/E3 spectrophotometric test indicated that the DOC extracted from the ECO manure-treated plots was more humified than that from the ECO control plots. There were no significant differences between the amended and control treatments in Olsen P or total P for all the tested cropping systems. Similar to that of the DOC response, the ECO cropping system that received animal manure recorded higher concentrations of water soluble P (P < 0.05). There was a linear relationship between DOC and water-soluble P concentrations of the soil:water extract across soils, indicating a positive relationship between them. To determine the effects of the different cropping systems on P bioavailability, a simulation model was used to study how changes in P soil chemistry may affect P uptake by crops. The simulation runs showed that predicted P uptake was greater for the animal manure based ECO and GF systems, but not for the legume based LER and PR systems (P < 0.05). This study shows that cropping systems that include animal manure may increase soil P availability.