Interactive comment on “ Three years of increased soil temperature and atmospheric N deposition have no effect on the N status and growth of a mature balsam fir forest ” by L . D ’ Orangeville

The research concerns a study on the effect of soil warming combined with realistic increased N deposition effects on the N status and growth of a fir forest. The study is well designed including replicates and controls. One interesting point is that the N addition was carried out on top of the canopies to mimic deposition effects, using a system of pumps and reservoirs. Surprisingly, little effect of warming and additional N depositions was observed on N availability and plant growth, questioning some of the conclusions from previous studies. I suggest acceptance of the paper, even if the air drying of the samples before extraction may have really affected the quality of the


Geoscientific Instrumentation
Methods and Data Systems Interactive comment on "Three years of increased soil temperature and atmospheric N deposition have no effect on the N status and growth of a mature balsam fir forest" by L. D'Orangeville et al.
used are not responsible for these results. I believe the authors should discuss more about the implications of 1) using heating cables vs. infrared warming, 2) heating only during the growing season (i.e. Apr to Oct) vs. year-round, 3) having two years of temperature/precipitation anomalies in their data set (2010)(2011), 4) the heating system malfunction in August 2009, 5) not having sampled the mineral soil by soil horizons (the authors sampled the first 30cm of the mineral soil, not taking into account the variable depth of the Ae and B-hor, that could have bias their results). I would also like to see a graph of the evolution of soil temperature for the control, fertilized, heated and fertilized+heated plots during the monitoring period.

Implications of using heating cables
The burial of heat-resistance cables is a widely used method to warm soils in climate change studies (see Aronson & McNulty, 2009, Agr Forest Meteorol 149:1791-1799. In comparison to infra-red lamps (IR), heating cables provide a more homogeneous warming of the soil strata (Aronson & McNulty, 2009). Considering the thickness of the organic layer, we cannot assume with certainty that IR lamps would have much of an effect on the mineral soil temperature unless a much larger area than the sampled area had been equipped with IR lamps which would have made it prohibitive. Given that one of our major objectives was to quantify the effect of soil warming on N availability in both the organic and mineral soil, we believe that using heat-resistance cables was the most appropriate method. The main disadvantage of heat-resistance cables is the soil disturbance caused by the burial of cables (which necessitates the use of non-heating buried cables for control trees). We would like to emphasize that great care was taken during the installation of the cables to minimise disturbance. Large roots >5 mm were never cut: the cables were passed below or above. Most of the disturbance probably impacted fine-roots, which have a relatively rapid turnover of ca. Three negative control trees (no soil disturbance) were monitored during the experiment, although these results are not presented in the manuscript. We measured radial growth using wood microcores (same method as in the manuscript) as well as foliar nutrient content. There were no significant differences between negative control and treated trees in terms of growth or foliar nutrients (D'Orangeville et al. [accepted]. A three-year increase in soil temperature and atmospheric N deposition has minor effects on the xylogenesis of mature balsam fir. Trees -Structure and Function). From these data, we assume that our results were not confounded by the initial soil disturbance caused by the burial of heat-resistance cables.
The manuscript will be changed in order to justify the use of heat-resistance cables as well as to clarify the role of cable burial in explaining our results.

Implications of heating during the growing season only and not all year
With the current climatic models for 2050-2090, it is predicted that the soil will continue to be insulated by a thick snowpack and soil temperatures are expected to vary very little in winter in the future (Houle et al., 2012, cited in the manuscript). Soil warming in winter would therefore not be an appropriate way to duplicate future temperature regimes in winter. But warmer spring temperatures are expected to induce an earlier snowmelt. The soil warming treatment was therefore applied in order to reproduce as precisely as possible, future soil temperature projections.

Implications of temperature/precipitation anomalies in data set
The term "anomalies" does not really apply here. precipitations from May to September averaged 654 mm, with lower and upper confidence limits (95%) of 456 and 851 mm, respectively. In comparison, the 596 mm of precipitations received in 2010 during that period falls within the normal range of precipitation at the site. In fact, 10 out of the 34 monitored years (29%) received less rain than during the May to September 2010 period. As for the 2011 year, the 747mm received during that period are also within the normal range at the site. 18% of the monitored years from 1975 to 2008 received more rain than in 2011. Thus, although we highlighted in the manuscript climatic differences between the two monitored years, both remain representative of the normal climate at the site. So there was no "anomalies" that could have affected the results of our study. We will change the manuscript to clarify this point, according to your comment.

Implications of the heating system malfunction in 2009
The heating was ended prematurely in 2009 in mid-August due to a malfunction of the power supply. The plots were nonetheless heated from late April to mid-August, while tree growth was terminated on August 10. The heating treatment was applied without any problems in the two following years. Thus, there is no evidence that this interruption may have affected our results over a three-year experiment, particularly when only the data of the second and third year are discussed.
5. Implications of not having sampled by soil horizon: It must be said that the Ae horizon at the site of treatments is discontinuous and very thin when present. Therefore, we did not sample the Ae (eluvial) horizon but rather the first 20 cm of the B horizon. The text will be changed to clarify this point.
6. I would also like to see a graph of the evolution of soil temperature for the control, fertilized, heated and fertilized+heated plots during the monitoring period.
We will add a figure (attached) displaying soil temperature per treatment for the two years discussed in the manuscript.