Residual Impact of Raw and Composted Poultry Litter On Soil Carbon Pools.

Application of animal manures or composts to soils increases soil carbon levels and improves soil physical properties.  However, there is little information on the duration of these effects after manure or compost applications cease.  We evaluated the four-year residual effects of applying poultry litter (PL) and composted poultry litter (CPL) at < 4 Mg ha^-1 application^-1 on soil carbon fractions and other soil properties in low input crop rotations.  We sampled soil in 2001, which was four years after PL and CPL were last applied in wheat-soybean-corn rotations fertilized with 1) NPK mineral fertilizers (MF) only, 2) PL supplemented with MF, and 3) CPL supplemented with MF.  Soil bulk density was greater in MF than in PL and CPL systems compared to initial values (P < 0.10) four years after PL and CPL were last applied.  There were no differences among systems in total soil C and N or in active and slow soil C pool sizes four years after PL and CPL were last applied.  The size of the slow C pool, however, was positively related to C input levels during the years that PL and CPL were applied, indicating that there was a 4-year residual impact of C inputs on the slow C pool.  Also, active and slow C pool rate constants were greater in PL than in MF systems (P < 0.05), averaging 0.2870 and 0.0053 d^-1 compared to 0.1103 and 0.0037 d^-1, respectively.  Rate constants were intermediate in CPL systems, averaging 0.2398 and 0.0045 d^-1.  We conclude that soil quality benefits of low rate applications of PL and CPL can last up to four years after PL is last applied and that these improvements should be considered when valuing PL and CPL as soil amendments and as sources of fertility.