Carbon accumulation in loblolly pine plantations is increased by fertilization across a soil moisture availability gradient

Silvicultural practices, particularly fertilization, may counteract or accentuate the effects of climate change on carbon cycling in planted pine ecosystems, but few studies have empirically assessed the potential effects. In the southeastern United States, we established a factorial throughfall reduction (D) × fertilization (F) experiment in 2012 in four loblolly pine (Pinus taeda L.) plantations encompassing the climatic range of the species in Florida (FL), Georgia (GA), Oklahoma (OK), and Virginia (VA). Net primary productivity (NPP) was estimated from tree inventories for four consecutive years, and net ecosystem productivity (NEP) as NPP minus heterotrophic respiration (RH). Soil respiration (RS) was measured biweekly-monthly for at least one year at each site and simultaneous measurements of RS & RH were taken five to eight times through the year for at least one year during the experiment. Reducing throughfall by 30% decreased available soil water at the surface and for the 0–90 cm soil profile. Fertilization increased NPP at all sites and D decreased NPP (to a lesser extent) at the GA and OK sites. The F + D treatment did not affect NPP. Mean annual NPP under F ranged from 10.01 ± 0.21 MgC·ha−1·yr−1 at VA (mean ± SE) to 17.20 ± 0.50 MgC·ha−1·yr−1 at FL, while the lowest levels were under the D treatment, ranging from 8.63 ± 0.21 MgC·ha−1·yr−1 at VA to 14.97 ± 0.50 MgC·ha−1·yr−1 at FL. RS and RH were, in general, decreased by F and D with differential responses among sites, leading to NEP increases under F. Throughfall reduction increased NEP at FL and VA due to a negative effect on RH and no effect on NPP. Mean annual NEP ranged from 1.63 ± 0.59 MgC·ha−1·yr−1 in the control at OK to 8.18 ± 0.82 MgC·ha−1·yr−1 under F + D at GA. These results suggest that fertilization will increase NEP under a wide range of climatic conditions including reduced precipitation, but either NPP or RH could be the primary driver because F can increase stand growth, as well as suppress RS and RH. Moreover, D and F never significantly interacted for an annual C flux, potentially simplifying estimates of how fertilization and drought will affect C cycling in these ecosystems.