Short-term nitrogen-15 recovery vs. long-term total soil N gains in conventional and alternative cropping systems

Cropping systems receiving organic N additions may sequester more N than conventional systems receiving inorganic N inputs. A short-term study using 15N-labeled vetch (Vicia spp.) and 15N urea was conducted to determine whether 9 years of organic N inputs to a cropping system led to lower short-term 15N losses than a cropping system under conventional management. The three cropping systems were: a conventional, receiving 15N urea, a low-input receiving 15N urea or 15N-labeled vetch residue, and an organic receiving 15N-labeled vetch residue with unlabeled poultry manure. Nitrogen recovery was calculated as the amount of 15N recovered in maize (Zea mays L.) and in the soil. Total N recovery from 15N sources in the three cropping systems were similar after one growing season and unaffected by the form of 15N application, whether organic or inorganic. Similar results were reported from other studies conducted at Rodale (USA), Rothamsted (UK) and Cadriano (Italy) where short-term N recoveries from organic or inorganic 15N sources were similar in conventional and legume or manure based cropping systems. Therefore, short-term 15N recoveries appear to be unaffected by the form of 15N application and whether organic or inorganic N-based systems are evaluated. However, results from the short-term 15N tracer experiments did not reflect known long-term trends of increased total soil N in the organic N-based cropping systems. The apparent discrepancy between short-term 15N recoveries and long-term gains in total soil N may be due to differences in total N inputs between the various cropping systems or the inability of short-term 15N studies to accurately reflect the long-term N retention of various cropping systems.