Effects of Organic Matter Amendments

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Animal manures and compost can be valuable nutrients sources to crops. In addition, while their amendments to soils represent a convenient disposal and recycle of considerable amounts of wastes, they allow to limit the application of mineral fertilizers and, thus, save farm money and energy. A number of studies have shown that manure addition is beneficial to soil in terms of plant productivity and soil quality (Haynes and Naidu 1998; Edmeades 2003).

A long-term experimental platform exists at the TO site in close proximity of the MESCOSAGR experiment, where organic fertilizers are tested. This platform was intended to evaluate management options of livestock farming in terms of crop production, soil quality, and environment impact. Different maize-based cropping systems are fertilized with bovine farmyard manure or slurry, in comparison to urea. It was therein found that tested organic fertilizers made N available to crops to the same extent as urea and were better retained in the soil leading to minor losses

(Zavattaro et al. 2011). Grignani et al. (2007) showed a lower N efficiency of farmyard manure than that of slurry when supplied at low rates, probably because the available N in farmyard manure was not sufficient at early growth stages. On the contrary, farmyard manure was utilized as well as slurry or even more efficiently (depending on cropping systems) when applied at high rates.

An investigation on chemical and biochemical indicators of soil quality was conducted on a selection of treatments. Farmyard manure was found to increase SOM content, potential mineralizable N and soil microbial biomass. Furthermore, application of farmyard manure was found to store the maximum amount of C and N per unit of C and N received throughout the experimental period. An investigation on soil respiration, under controlled conditions, confirmed that C supplied with farmyard manure was more stable and less decomposable than slurry (Monaco et al. 2008). These authors reported that use of slurry resulted in a lower accumulation of C and N in soil per unit of added C and N than for farmyard manure. With respect to the treatment receiving urea, the liquid manure applications did not change the amount of total N in the soil, but did significantly increase the fraction of easily mineralizable organic N. Data modeling leads to quantify C retention into SOM for 0-30 cm tilled layer. Following these data, after the first year from field application, 26% of slurry-C and 46% of farmyard manure-C is still in soil. Increasing amounts of organic fertilizers enhanced SOM. Conversely, our results showed that increasing amount of urea did not produce any significant difference in SOM content, indicating that the amount of mineral N supplied was not determinant for C sequestration (Bertora et al. 2009).

A research on N mineralization in the same treatments revealed that the time and extent of net N mineralization and plant N uptakes were not affected by fresh manure application. Instead, the effect of past management increased the maximum net N mineralization rate obtained with farmyard manure (Monaco et al. 2010).

Field experiments made in southern Italy in the same site as that of Mescosagr (Fagnano et al. 2011) showed a favorable effect on lettuce yield of compost made with municipal solid wastes (MSW), proving that compost fertilization may have agronomic and environmental benefits in sandy-loam soils, if amendment rates are tuned to N requirements of crops. The compost rate of 30 Mg ha-1 satisfied the N requirements of two lettuce cycles, without causing surplus of nitrogen in posthar-vest periods and dangerous levels of nitrate and potentially toxic elements in soil and plants. Furthermore, compost fertilization proved to compensate SOM degradation due to cultivation, estimated in 6 Mg ha-1 of C, but was also able to significantly increase SOM content (2.7 Mg ha-1 of C), through C fixation in stable SOM. A study about the effects of such compost on the molecular changes in soil organic C confirmed the formation of more stable C compounds, such as fatty acids, n-alkanes, and various biopolyesters derivatives (Spaccini et al. 2009).

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