Manure fertilization increases soil respiration and creates a negative carbon budget in a Mediterranean maize (Zea mays L.)-based cropping system

Agronomic research is important to identify suitable options for improving soil carbon (C) sequestration and reducing
soil CO2 emissions. Therefore, the objectives of this study were i) to analyse the on-farm effects of different
nitrogen fertilization sources on soil respiration, ii) to explore the effect of fertilization on soil respiration sensitivity
to soil temperature (T) and iii) to assess the effect of the different fertilization regimes on the soil C balance.
We hypothesized that i) the soil CO2 emission dynamics in Mediterranean irrigated cropping systems were mainly
affected by fertilization management and T and ii) fertilization affected the soil C budget via different C inputs
and CO2 efflux. Four fertilization systems (farmyard manure, cattle slurry, cattle slurry + mineral, and mineral)
were compared in a double-crop rotation based on silage maize (Zea mays L.) and a mixture of Italian ryegrass
(Lolium multiflorum Lam.) and oats (Avena sativa L.). The research was performed in the dairy district of Arborea,
in the coastal zone of Sardinia (Italy), from May 2011 to May 2012. The soil was a Psammentic Palexeralfs with a
sandy texture (940 g sand kg−1
). The soil total respiration (SR), heterotrophic respiration (Rh), T and soil water
content (SWC) were simultaneously measured in situ. The soil C balance was computed considering the Rh C
losses and the soil C inputs from fertilizer and crop residues. The results showed that the maximum soil CO2 emission
rates soon after the application of organic fertilizer reached values up to 12 μmol m−2 s
−1
. On average, the
manure fertilizer showed significantly higher CO2 emissions, which resulted in a negative annual C balance
(−2.9 t ha−1
). T also affected the soil respiration temporal dynamics during the summer, consistently with results
obtained in other temperate climatic regions that are characterized by wet summers and contrary to results
from rainfed Mediterranean systems where the summer SR and Rh are constrained by the low SWC. The sensitivity
of soil respiration to temperature significantly increased with C input from fertilizer. In conclusion, this research
supported the hypotheses tested. Furthermore, the results indicated that i) soil CO2 efflux was
significantly affected by fertilization management and T, and ii) fertilization with manure increased the soil respiration
and resulted in a significantly negative soil C budget. This latter finding could be primarily explained by a
reduction in productivity and, consequently, in crop residue with organic fertilization alone as compared to mineral,
by the favourable SWC and T for mineralization, and by the sandy soil texture, which hindered the formation
of macroaggregates and hence soil C stabilization, making fertilizer organic inputs highly susceptible to
mineralization.