Herbaceous layer development during spring does not deplete soil nitrogen in the Portuguese montado

Nitrogen (N) content in the soil and in the herbaceous biomass were monitored during spring of 2004–2006 to determine how the herbaceous layer development influences soil N availability in the montado ecosystem of southern Portugal. Highest (246.6 ± 52.7 g m−2) and lowest (123.2 ± 89.5 g m−2) peak biomass occurred in 2006 and 2005 respectively. Total soil N within the top 20 cm soil profile ranged between 0.2 ± 0.1% in February and 0.41 ± 0.2% in May, while available soil N was lowest (5 ± 2 μg g−1soil) in February but increased three-to-five fold in March and was >17.5 μg g−1soil at senescence in May. Significant (p < 0.001) increase in total N in the aboveground pool occurred between February and May. There was however, no decay in soil N content. Instead, the herbaceous vegetation enhanced soil N input and N retention in the ecosystem. Most of the herbaceous plants were annuals with large reserves of organic N at senescence, which returned to the soil as detritus. The herbaceous vegetation is a critical component of the montado that contributes to N recharge and cycling within the ecosystem.

Research highlights
► Annual aboveground biomass of 200 g m−2 is produced in the Portuguese montado.
► About 4–7.4 g m−2 of nitrogen (N) resides in the herbaceous biomass pool.
► Soil N pool is not depleted by herbaceous vegetation uptake in spring and summer.
► Spatial and temporal root and soil N distribution facilitate efficient N uptake.
► The herbaceous layer retains N as detritus and prevents its loss in winter rains.